Oh boy, I had too much fun writing the descriptions for those nightmares, so there's some creepy stuff coming up ahead :)
As always, I'm terribly sorry if anyone seems a bit OOC in certain moments.
This chapter was a REAL writing hell, and I'm still not happy with it. Expect this particular chapter to get rewritten over...And over...And over...

"What is going on? Why am I aboard the Blue Typhoon again?"
Tails was having another one of those nightmares. Specifically, a nightmare about a particularly painful day. The day of the final battle against Dark Oak. The memory of that particular day always cut Tails like a knife - how Cosmo used her power to weaken Dark Oak, how he had to fire the Sonic Driver to finish off Dark Oak, killing Cosmo off together with him, and how he cried in complete misery as he realized just what he had done. But this time, the memory was completely different. Dark Oak was nowhere to be seen – in fact, there wasn't anyone else in the cosmic void other than Cosmo in her normal form, who was seemingly standing out in the void and begging for mercy: "Please, Tails, don't do this! This isn't the way to end this conflict!"
If Tails had a choice in the situation way back then, four years ago, he sure wouldn't ever try to hurt Cosmo in any way. However, Tails had no other choice – it was either the life of one person, or the entire universe, with no third option. In this nightmare however, Tails didn't press the button to fire the cannon on his own accord – this time, he pretty much felt his hand reach towards it against his will. As much as he struggled not to do this, his hand just ignored the orders from his brain completely. And as the young fox pressed the button and saw Cosmo get disintegrated into nothingness, he heard voices. Millions upon millions of voices, berating him on his actions...
"You monster!"
"How dare you?!"
"Why did you do this to her?!"
"That was completely heartless!"
"I'm sorry…I had no other choice! There was no other way! I didn't want to do this, I swear! " – Tails shouted, but his words were pretty much going into nowhere. Moments after that, Tails realized he was no longer aboard the Blue Typhoon, instead standing in the middle of complete darkness. At first, it was eerily quiet, and Tails felt completely alone…Until suddenly, the voices had returned, except shouting even more often, still accusing Tails of what he did back then...
"Murderer!"
"You killed her!"
"Traitor!"
"How could you do that to her?!"
"You heartless monster!"
"I hate you!"
"Soulless freak!"
In what felt like an eternity, the voices had devolved into nothing more than a screaming mess, which became too overwhelming for Tails. And so, the fox jolted awake, screaming as he felt himself getting overwhelmed by fear and guilt.

It was about 1 AM at the moment. The room where Tails slept in was almost pitch-black, so he reached to switch the desk lamp on and stood up from the bed to run off into the bathroom and splash some cold water on his face. This certainly wasn't the first time that he had these nightmares since the day of Cosmo's return - in fact, he already had nightmares for quite a few nights straight, and days before Cosmo's return, his sleep was usually dreamless - not exactly pleasant, but at least he wasn't having so much trouble sleeping. But now, after Cosmo's return, the nightmares he had in the past had returned, almost depriving the young fox of his sleep.
As Tails exited his bedroom, he practically bumped into Cosmo. After the plantgirl got back up, she asked:"You're having trouble sleeping too?" Tails immediately turned to face away from her and said: "Yeah, sort of. Had another bad dream, so I need to splash some cold water on my face." – With that said, Tails entered the bathroom while Cosmo stayed outside. Moments after, Tails came back, wiping his face with a towel.
"Tails, is something bothering you? I don't know if you had these nightmares before my return, but..."
"No, no, I'm fine. I just...Have some trouble sleeping, that's all. Go back to your bed now, I don't really want you to stay up all night because of me." - With that said, Tails hastily ran off back to his bedroom. Cosmo stood before the bedroom door for a few moments, and just about reached to knock on it, but soon pulled her hand back and said to herself: "He is clearly having some trouble sleeping. I better ask him what's the matter in the morning." - As Cosmo said that, she had walked back to her bedroom, and then fell down onto her bed. The young seedrian had lied there for a few moments before she herself fell asleep.

As for Tails, he had some trouble going back to sleep. The previously calming silence of his bedroom now felt completely eerie, and Tails didn't even want to fall asleep, trying his best to stay awake after the nightmare. But soon enough, the young fox had fallen asleep. This time however, the night was almost entirely dreamless.
At about early morning, Tails had woke up quietly. He looked around for a while, not seeing anything unusual. He had exited his bedroom quietly, heading for the bathroom to brush his teeth. Just as he finished and washed his face, his reflection suddenly said in an eerie voice: "Murderer."
After that, Tails had woke up for real. The young fox slowly stood up from his bed and ran into the bathroom, and this time everything was going normally, without anything out of the ordinary happening - his reflection didn't talk to him, there wasn't anything out in the window, everything was alright. After Tails had finished his morning routine, he ran downstairs, thinking about the nightmares he had overnight.
"Of course, it doesn't take a genius to figure that one out…It's my subconscious constantly tormenting me and basically saying that I still am guilty for hurting her like this…But she is back with me again, she doesn't seem to be angry at me for what I done back then…And yet, I still feel guilty about this whole incident… Damn it, Miles, you need to get your mind off of it somehow. Just go to your workshop and get back to working on that plane upgrade. There is always room for improvement." – As Tails thought that, he didn't even notice how his legs pretty much carried him to the workshop. – "Well, since I am here already, it's time to get to work." – The fox said, sitting down beside the drafting table and looking over the blueprints of The Tornado all over again.
Meanwhile, Cosmo had already woken up, and when she exited into the corridor, she noticed that the door to Tails' room was left wide open, and it seemed that Tails himself was gone as well.
"Probably at his workshop. Okay, Cosmo, just ask him what's the matter. It shouldn't be that hard." – With these thoughts, Cosmo went to exit Tails' house and check on him in his workshop. Sure enough, she noticed that Tails was there, sitting beside the drafting table, hastily drafting a blueprint of some sort.
"Good morning, Tails." – Cosmo said, approaching Tails.
"Oh..." - Tails had turned to face away from the drafting table as he heard Cosmo greet him. - "Good morning, Cosmo. Did I miss breakfast?"
"No, it's not about that. You seemed a little worried last night. What's the matter?" - The young plantgirl asked, approaching Tails.
"Eh, it was just some nightmares. Nothing worth your attention." - The answer did seem a bit off-handed, as Tails had immediately went back to working on his blueprints after he said that.
"Very well then. I'll come in when breakfast is ready. You want pancakes as usual?" - This wasn't the first time Cosmo had to cook something for Tails, since he was often busy in his workshop and didn't have time to cook anything himself. Even though Cosmo didn't yet know how to cook some of the more complex meals, the more simple ones proved to be just enough, if a little repetitive.
"Sure thing." - As Tails said that, Cosmo had quietly ran off back to their house. Even when Cosmo was cooking, though, her thoughts were still occupied with Tails' current state. "Considering what he went through, it's understandable that he is suffering from nightmares. And it seems like he didn't want to talk about it so much. And yet, I feel that I must repay him for his emotional support back then." - As Cosmo kept saying her thoughts out loud, she didn't even notice how she almost overcooked one of the pancakes, and immediately hurried to remove it from the frying pan, later focusing on the cooking again, ending her trail of thought on the following: "If he wakes up from a nightmare in the middle of the night again, I'll just ask him what's wrong. I'm sure he trusts me enough to tell me what's the matter."
The rest of the day had gone by quite routinely - Cosmo was mostly either doing housework, cooking or reading some books, with a few romantic novels being the ones that caught her attention the most. As for Tails, he spend most of his time at the workshop, only leaving it to take care of some of his natural needs. The two didn't really talk much throughout the day, too - Cosmo was too afraid to seem clingy by once again asking Tails what' wrong with him, while Tails had trouble mustering up the courage to go on ahead to Cosmo and confess his problems. They didn't really say much to each other when they were going to sleep, either - all they really said to each other was an off-handed "Sweet dreams" as they went to their respective bedrooms.
"I'll just ask him what's the matter. I'm too worried about his mental well-being - he certainly didn't look like he had much restful sleep when I met him again as well. Yes, that's what I'll do." - With that in mind, Cosmo had soon fallen asleep. As for Tails, at first he struggled to fall asleep, still thinking that the nightmares might return all over again.
"Then again, this guilt will keep eating away at me if I don't tell her what's wrong. That's it, Tails. If the nightmares continue, you go on ahead, and tell her everything. That would be best for you." - And on that trail of thought, Tails had soon fallen asleep.

"Tails! Wait up!"
Cosmo had woken up in the middle of nowhere - it seemed like she was out in a grassy field of sorts, with no one around. Suddenly, she saw Tails not too far away, running off. Cosmo had stood up and ran after him, and soon noticed that he stopped in front of a cliff. A sense of dread had overcome her as she slowly approached the young fox. As soon as she was close enough to reach out for Tails with her hand, the fox had turned to face her, making her flinch back in fear - it had no face! After that shocking revelation, the strange apparition had jumped down from the cliff and into the endless abyss below, Cosmo shouting "Wait!"
But it was too late - the fox made no effort to get back up, letting the endless darkness consume him...

"Oh no…Not again…" – Tails mumbled as he found himself in the middle of nowhere again. This time, in his nightmare, he ended up in the middle of a rather strange grassy field – the grass was a fleshy color, the sky was completely black with blood-red clouds, and the air reeked of death.
"Now this is getting even creepier than before…"
"You can't escape your guilt, Miles." – A voice rang from behind Tails. The young fox slowly turned around and saw an apparition that looked a lot like Cosmo, except heavily mutilated – one of the buds was torn off with dark green blood oozing from the hole left in its place, the chest of the creature had a huge hole burned right through it, and instead of the beautiful blue eyes that the real Cosmo had, the apparition had empty eyesockets that oozed dark green blood. Tails immediately froze in place as he saw this monstrosity. – "What's the matter? Scared stiff?" – The monster said, taunting Tails.
"Please, just leave me alone…" – The fox replied, backing off.
"I told you already – there is no escaping your guilt. You will be stuck with your painful past forever, Miles."
"This isn't real…This is just a dream. A really bad dream, but still a dream nonetheless…" – Tails mumbled, still backing off. Suddenly, he realized that he has nothing to step back on – the ground below him had disappeared, and now was nothing more but an endless abyss. A twisted smile crossed the Cosmo apparition's face, as she grabbed Tails by his neck, lifted him above the chasm, and said in an eerie whispering tone: "Murderers must always get their punishment." – And with these words, the apparition let go of Tails, making him fall down the endless abyss. As much as Tails wanted to scream, he found out that he couldn't utter anything but a wheezing sound as he fell into the endless abyss…

"Oof!" – Tails had fell out of his bed. He slowly stood up, put the pillow back on, and went to exit the bedroom, but not before looking out of the window – the sun was already dawning over Mobius again, and another morning came onward. After looking out, Tails headed out of his bedroom and into the corridor. – "I must tell her about the nightmares that torment me. I see that they are directly linked to her, so Cosmo is the only one who can at least try to help me get some restful sleep for once." – With that said, Tails had slowly walked over to Cosmo's bedroom, and was just about to knock on the door, but stopped at last second. – "No, I better not wake her like this. She was probably busy keeping the house clean while I was away and needs her well-deserved rest after yesterday."
What Tails did instead was go downstairs, but instead of heading for his workshop, he decided to just wait until Cosmo comes down as well. Sure enough, the young plantgirl had soon ran downstairs and into the living room, yawning. As soon as she had noticed that Tails was waiting in the living room, she had ran over to him and said: "Tails, I got to ask you something."
"No, it is I who should tell you about something..."
As much as Cosmo wanted to speak up first, she had felt that Tails might just want to confess what's wrong right now, so instead of insisting that she speaks up first, the young plantgirl replied: "Okay, go on."
"It's about those nightmares I kept having every night." – Tails looked down as he said that.
"I knew it. So, what is bothering you?" - Cosmo had asked in a rather concerned voice.
"They all seem to be linked to…A certain incident from back then, four years ago. You know what incident I am referring to."
And afterwards, Tails told her everything about the nightmares that he had lately. As he kept talking, Cosmo was getting more and more terrified – it wasn't the descriptions of Tails' nightmares that scared her, but how vivid these descriptions were, which implied that these nightmares must have really affected Tails.
"I just cannot let this guilt go. I hate myself for what I had to do to you. And I understand if you had any hard feelings towards me back then." – Tails covered his face with his hands after he said that. Cosmo went silent for a few moments, thinking about what Tails just told her. Soon enough, she had grabbed on to Tails' hands, moved them away from his face, and hugged him. The young fox was confused for a few moments, asking: "Cosmo, what are you…"
"Don't worry, Tails. I wasn't angry at you back then, and I'm not angry at you now. You made the right choice – if you didn't do this, we wouldn't live to see each other again, right?"
"I…I guess you have a point." – Tails hugged Cosmo back as he said that. – "There is no point dwelling on the past – we are together once more, and there won't be anyone who will separate us again."
"You're a lot more like the cheerful fox I remember you being." - The young plantgirl replied, setting back and smiling as she looked into Tails' eyes. - "So, I suppose that inner conflict is resolved and you won't be having nightmares anymore?"
"I suppose it is. Now that we had gotten this out of the way..." - Tails heard his stomach rumbling and didn't quite finish his sentence, asking after a few moments: "You know, I think we can talk about the rest later. I'm hungry for some pancakes right now."
"In a few moments!"
As Cosmo ran off to the kitchen, Tails had breathed a sigh of relief. There was no doubting it - his past shall be long bygone now, left as nothing more than distant memories.