Termination is the the brother of Protector Sans. Who wants to kill everyone so they don't have to face the "End".
Termination and Protection Sans are a Sans and are Sanes, let me explain. So when a Sans (Or any Undertale or AU character) is born they are sent to their timeline, but if they are "Different" they are stuck in the Omega Timeline. Well when a Sans was being made its first and its main cell was duplicated and then they fused, then they duplicated again. So there were 2 cells, 1/2 of each were the original and 1/2 of each were the duplicated version. So none of them are the "Real version". Then since they were "different" they were let to be kept in the Omega Timeline. And one grew up to be known as Terminator Sans and the other protector. The reason they are so special is because they were wandering and were randomly teleported to the Ultra God TOBY FOX and he gave showed them "The End" or some may say the death of matter, energy, nothingness, thought, deletions, and death itself. Anyway one of them was nicknamed Termination Sans because he want to kill everyone himself so they would have to face the "End". And the other one wants to protect everyone and is trying to stop the "End". And after Termination Sans went out to kill the first one he did say he was sorry and he was very nervous. But after killed him he changed he WANTED to kill people to but only to not put them through TRUE PAIN. And the HATE each other and if they tell anyone what the "End" is... well lets just say it's their end. And the only thing that has a chance against the "End" is if Error, Ink, Terminator, Protection, Error404, S.T, All the other gods, and Classic Sans fuse. But Terminator refuses to fuse.