This story is inspired by the premise of the fanfiction Residuos de lealtad by c0pitos. I recommend you read it. Although it's in Spanish, you can always translate it.
RWBY and Overlord are owned by Rooster Teeth and Kugane Maruyama, respectively.
The Sorcerer Kingdom governs over almost every land. There is no intelligent being who doesn't know that. There are small groups of resistance that are of no significant concern. How could they? If their creation was planned by the Sorcerer King in his infinite knowledge. Giving them the illusion of an alternative that, in the end, is also controlled by the person they are against.
The New World, as the King sometimes refers to the land, is now a pacific place, a small piece of heaven for those who follow the laws. Food, work, and health. Kids can run free and learn whatever they want. The Adoration for the King is almost religious, and in some cases, it is sacred, thanks to the Faceless Preacher.
The security inside the kingdom had made the adventurers start to do what their name implied: go on adventures. Find new places, move to lands that are not on the maps, and get connections with new species of monsters and humanoids. Always with the idea of a pacific contact, and in case of any hostility, they will defend themselves and return to inform the kingdom. From there is the King's decision to attack or try to take over pacifically.
The adventurers worked hard, as they loved the bonuses they got when they found new civilizations, either the ruins or an active one.
Some idiots had initiated hostilities and tried to sell the information of the place as a ruin. Still, with a simple spell of one of the King's delegates, they discovered the truth. The punishment they go through is unimaginable. It is a fool's decision to go against the rules the King had given the adventurers.
With this in mind, a brave man appeared before the Sorcerer King. "Oh, wise king, Ainz Ooal Gown-sama. It is a great honour to be in front of you", said the man kneeling. The man had a strong body and short brown hair with some grey here and there.
Ainz-sama sat on his throne and only made a small gesture with his hand, asking him to continue.
"Thank you, your majesty. I came here to ask for your permission to go on an adventure with my ship. I am planning to cross the world, move from the ports of the extinct Re-Estize, and arrive at the ports of the Kingdom of Soba".
The King looked at the man, and a shiver passed through the adventurer. The King then talked, "And why are you giving this information to me and not to the adventurers' guild master? Is there any problem?"
The man looked to the floor, a little afraid. Still, he answered, "No, Your Majesty. He has been very helpful. But it is told by the sailors that there are sea monsters that we cannot defeat. If… If is not too much to-to ask. If you allow us". The man was finally going to tell his real motive and could not help but stutter. It did not help that the King leaned to the front.
After making up his resolution, the man looked into the eyes of the King and said, "I would like to ask Your Majesty to lend me one of your death knights to defeat those monsters".
Ainz-sama leaned back and caressed his chin with one hand. The man looked at him and thought that the King was thinking, and he decided to tell him everything he had in mind. All the study he made with his friends so the King could help him to grant his wish. He was only doing this because he wanted to cross the world and be the first man to do it. "Your majesty, I know that right now you are concentrating your forces for a possible attack on the Draconic Kingdom, but if my plan works, we can pincer them. The Tsar of Soba Kingdom is known to have bad relations with their neighbours; they could aid us, and if not, they should be easy to defeat as they are under too many enemies. We can-"
"I don't mind", said the King and the man shut up. "I don't mind letting you one or two of my death knights", said the King solemnly.
After that, the man bowed repeatedly to the King and left the room. He was later sent to one room to get the detailed information.
The King then relaxed. To him, creating a death knight was too easy, and he had too many of them. Indeed, the war against the Draconic Kingdom is happening, but the approach they are giving them is different. Although the fight that occurred in the air was a fantastic sight. Something that no one has seen before. Close to the World War, flight attacks from his old world, but with Dragons. Shalltear made a fantastic decision when she suggested using riders with the Dragons as if they were aeroplanes and killed her opponents. The raider could aid the Dragon with defensive spells, while their opponent could barely do anything.
But the recent petition made him remember something from his childhood, something he was told when he was at school. Still, he is unable to remember what exactly. "Mh, I forgot to ask for the name."
After his first contact with the King, the man started to describe the mission, which was sponsored by the Sorcerer Kingdom. He then put his information in the adventurer's guilt. And after 3 months, he had everything ready. People were in the ship, the supplies were already inside, and the Death Knights were waiting.
"My name is Cristobal. I will thank all of you who decided to part take in this new adventure. Let me be clear: I don't care if you are a dwarf, an elf, a human, a male or a female. You are here to work and make this successful," said the man with a smile.
The number of dwarves was more significant than the humans or the elves. Their sense of adventure grew after the association of the Dwarf Kingdom with the Sorcerer Kingdom. The elves have started to blend even more, thanks to the reforms created, which affected the Baharuth Empire, in which elf slavery was abolished unless it was part of a punishment. When the Elf Country fell, the remaining elves moved to the Sorcerer Kingdom and started to experience a better life. However, the elves that are now on the ship were ex-slaves who had experienced the benefits of the kingdom and some half-elves.
Inside the ship was a tall construction to hold the death knights. It looked like a simple wood house, but it protected them from various ship movements. They were in a specific place to go out and help the crew in case the sea monsters appeared, and it was also in a particular point to give the ship stability.
The ship set sail for adventure, with all the crew having confidence. The crew felt that nothing could go wrong, and this was the protection they felt for the Death Knights. It was a recurring talk. Only a few years ago, they would have been scared to have the death knights too close to them, but now, it was a relief.
Ife, an elf, was on night duty. She was over the main mast, while her new best friend, and probably something more, a dwarf named Sabion, was on the steering wheel, standing over some boxes so he could see.
The elf saw a storm far away and would let them know when something caught her attention. A flying object fell into the ocean and made waves that shook their ship.
"Hey! Nice ass, what was that?!" screamed Sabion.
The elf was grabbed off the mast, but she was still watching the direction of the object. She grabbed a binocular given to her by the adventures' guilt. She could inspect the thing, and when she realized what was happening, she turned to the dwarf, "A ship! There are survivors on the ocean, southwest!"
Sabion was behind the steering wheel, but he wasn't the Capitan. He should ask the Capitan who would be in his position. At that moment, he decided to change direction to get closer.
While changing direction, another person arrived. "What in the world happen?" It was Cristobal.
"Not sure, but it seems that there was a ship wracked close to us. We are heading to rescue the survivors", said Sabion.
The ship Capitan got close to the edge of the ship and saw that they were getting closer to a bunch of wood floating in the ocean. He nodded, got back inside to wake the crew, and asked them to help the survivors.
It took them around half an hour to rescue the survivors, just six people. All of them had some animal features, so they assumed they were part of the beastmen.
After being rescued, they were taken inside and had hot tea in the dining room.
"Thank you for rescuing us", said a man with ears like a dog. The rest of the survivors were also thanking the crew.
Cristobal nodded, "no problem. But what happened to you all?"
"Grimm happened. We are just a small fishing group, but the storm and the giant Grimm appeared." Answered the dog man.
"Grimm?" asked Cristobal.
"Yes. It was the biggest Grimm I had ever seen. It was like a giant octopus, and with one of their tentacles, it grabbed our ship and sent it to the air like nothing", said the man.
"A giant octopus. Those could be the monsters of the sea…" wondered aloud the Capitan.
"Monsters of the sea?" asked in a low voice the dog man.
"Oh, sorry. I have not introduced myself. My name is Cristobal. We are currently on a special recognition mission on the sea."
The other man nodded, "A pleasure. My name is Leo, I am from Menagerie."
Both men shook hands, both having similar thoughts. Cristobal had never heard of Menagerie and believed it was part of the Beastmen Country. Leo thought the ship was weird and with an old style, maybe from Vale, seeing many humans and Faunus together, although it was strange that all of their Faunus had the same features.
Suddenly, the ship started to shake, and a dwarf entered the room, "we are under attack! A giant black octopus is attacking us!"
The rescued people started to shake, but the ship's crew did not hesitate and went outside. Then they saw it. The tentacles were the only thing that could be seen, and currently, it had been contained because some elves had used magic to defend the ship.
The Capitan took his sword from his hips and asked, "How strong is it? Can we kill it?"
"We can stop him for now, but I don't think we can kill it", answered one of the dwarves.
"FUCK!" screamed Leo when he stood outside. He saw the Grimm and was terrorized by it. He stood there, with a sword in hand, trembling.
Cristobal got close and said, "Don't worry, we have a secret weapon".
Then he opened the door so the Death Knights could get out. Cristobal screamed to them, "Go and protect us from the monster!"
The eyes of the Death knights shone in red, and they walked outside their rooms. Leo fell to his knees when he saw them. A monster in the shape of warriors, with weapons in their hands, and unlike any other Grimm, the monsters in front of him did not have a white mask.
The Death Knights roared, and Leo soiled his pants.
The Grimm attacked, but the tentacle was cut clean when a Death Knight jumped with a sword. Unfortunately, the tentacle still hit one of the masts. It would fall over Ife, who was already on the floor due to the ship's movement. She felt her death close, and then, as if in slow motion, she also saw Sabion covering her body with his. Both closed their eyes, waiting for their death… a death that never came. Instead, the other Death Knight held the mast with one hand and moved it behind its arm.
The other Death Knight attacked and cut another tentacle, making the Grimm's head appear. A white mask that covered the lowered part of the head. The beast's scream was a high-pitched squeak that made everyone cover their ears except for the Death Knights, who made a guttural roar against the Grimm.
The Grimm started to throw things at the ship, but the Death Knights used their shields. That kept repeating until one of the monsters created by Ainz-sama jumped and grabbed one of the tentacles.
The Grimm moved its tentacle and tried to eat the Death Knight, but using its sword, the Death Knight cut several tentacles. Unfortunately, it did not help as the undead fell into the Grimm's jaws. But then the other Death Knight jumped to the Grimm's head and stabbed with its sword multiple times, killing its opponent and then jumping back to the ship.
The crew celebrated until they saw that the monster had started to disintegrate. The whole thing was so much for them to understand, and at that moment, the Capitan decided. They were going back and reporting about the monster capable of killing one Death Knight.
He did not want to risk it and let its ship be destroyed. He does not have a way to prove the monster exists, but he does have beastmen with him, so he has testimonies outside his crew. He only hoped that this was going to work.
When the Capitan told them what they were going to do, he decided to have a long conversation with the beastmen, especially concerning this monster they called Grimm.