Rise of the Hallowed Mist.
Wasting no time, Gwen, Artoria and Nero left the barrier, protected by the power of the Hallowed Mist, and engaged Caligula in combat. First to arrive was Artoria, using her Excalibur to try and slash her enemy from above, but her attack was avoided.
Gwen and Nero then tried to pincer Caligula, closing his escapes from the sides. Both girls collided with him, Nero slashing his sides while Gwen attempted to cut one of his arms, but wasn't able to cut deep enough. Caligula then grabbed both of them by the head and pushed them against one another. Then, he threw Gwen towards Artoria, with the Saber having to take a moment to catch the Assassin before being knocked off her feet by having Nero thrown against her as well.
As they tried to get back up from being used as living projectiles, Gwen saw Caligula jump towards them in an attempt to crush them. Artoria reacted quickly at this and struck him mid air with Excalibur, sending him flying.
"Artoria, throw me!" - Gwen yelled while running at the Saber.
Understanding her friend's plan, Artoria put her hands in such a way that Gwen could jump on them, and after she did, the Saber pushed with all her strength and launched her friend into the skies, right towards Caligula.
Both figures met in the air, and the Berserker immediately tried grabbing the Assassin with his hands, trying and failing to reach her. As she watched Caligula struggle while falling, she prepared 4 of her needles and shot them against it, one for each limb of his. These were buried into its flesh and pinned him to the ground after it landed.
"Nero!" - with a yell, Gwen got the attention of the girl, who was prepared for whatever had to be done - "Stab him with your weapon, that will begin to cleanse him!" -
Looking at Caligula, her uncle, now just nothing more than a puppet of the Black Mist, like that, made her stop dead in her tracks. Even without knowing whether or not the creature in front of him was truly her uncle, or just a puppet made in his image…
"Nero… Please…" - if the situation wasn't conflicting enough, hearing her uncle's voice come out from that body, in a tone that seemed at both parts rageful and filled with pain, made it all the worse.
"Uncle… PLEASE FORGIVE ME!" - with a rage-filled yell, she plunged her sword straight into Caligula's chest. Or, at least she tried.
That single moment of hesitation before allowed Caligula to catch the sword on his hands, barely stopping it from fully piercing its chest.
Nero applied as much force as she could to try and win against it, but it simply wasn't enough. The bleeding Caligula mustered enough strength to kick Nero away and pull the sword from his chest.
It weakened him, but he wouldn't go down without a fight.
Nero rushed to pick her sword back up as Artoria and Gwen clashed their weapons against Caligula, managing to push him back as it roared against them, Black Mist pouring out of his mouth non stop.
Both Servants glanced at each other, and it only took that for them to coordinate a plan to bring Caligula down once more.
Gwen attacked first, coming from above with her scissors and launching an attack straight into Caligula's head. The Berserker tried to retaliate and punch her before she could hit it, but this was his mistake, as the Assassin was never aiming to his head. Instead, she dodged the hit and slashed his right leg, cutting it off.
Caligula let out a roar of pain as it launched an attack against Gwen, but she was faster and moved out of the way, just as Artoria came from behind and slashed its arm off, the light from her Excalibur alongside the power of the Hallowed Mist allowing her to cut through.
The Berserker tried to get back on up, fueled by pure nightmarish rage and bloodlust, but the damage was done. Without a leg and arm, it could do nothing to prevent Nero, who approached silently from behind, to cut off its head. The last thing that the mad Caligula heard was…
"I'm sorry" -
And with a clean swipe, he was gone. His body dissolved into the corrupting Black Mist and scattered through the air, as the dark moon above went away and allowed the sun to shine on once more. It had been a tough battle, but Chaldea managed to fend off the enemy forces, and could now have their much deserved break.
Soldiers from Rome rushed outside to form a perimeter after having been neutralized by Caligula's corrupted Noble Phantasm as Nero was carried back into the city by Artoria and Gwen, as the long fighting before this, as well as having to kill her uncle in such a state took a heavy toll on her mental state.
After Nero was put into her bed to rest, Gwen had a meeting with Ritsuka, Mashu and Roman to discuss their plans moving forward, especially with the fear of facing Servant fueled by the powers of the Black Mist more frequently.
"I can try to give my threads to everyone before a fight to help, so that way we can get a better chance if the Black Mist shows up again" - the Assassin spoke first.
"Wouldn't that be dangerous to you? We saw that it caused you pain to pull out even just a small piece of thread, and having to do it before every battle…" - Mashu said, worried about Gwen's health.
"I'll be fine, as long as it's to help my friends, I'll power through anything" - she said in an optimistic tone of voice.
"There'll be no need for that" - Roman answered to stop Gwen from hurting herself further - "I examined today's battle since the start, and I think I came up with a solution for this problems" -
"That's amazing Doctor! What's the plan?" - Ritsuka asked in excitement.
"I'd love to tell you, but sadly for the moment it's just a prototype, and I'll have to wait for Gwen to return to Chaldea in order to test if it works or not. However, if it does, then Chaldea will now have a way to fight back against the Black Mist" -
The Assassin was amazed at how Roman managed to figure something like the Hallowed Mist so fast, and she was ready to test it as soon as she and the others returned to Chaldea.
With this new invention, they were one step closer to thwarting whatever new plan Viego had for this world.