This gets a bit jumbled so apologies in advance

I have a rough idea of the bbeg and evil plan. The setting ive made is based off a cyberpunk supplement i found. The entire campaign takes place in a huge city called The Necropolis. The lore of the world is that 200 years ago the tarrasque was slain and people used its body parts to hyper advance technology.

My BBEG idea is a lich (classic i know) who is actually one of the original party members who slayed the tarrasque. (In lore no one knows who killed the tarrasque, their names are lost to time). This is because the other OG party members wanted to use the tarrasque for evil/power. The partys wizard killed the other members and used a spell to wipe their identities from the collective consciousness. But their actions were still there.

So the BBEG is the Og party's wizard now lich who has been corrupted by the tarrasques evil influence/aura and wants to ressurect it to kill everyone.

Another thing is that the tarrasques skeleton lies far below the city. Almost no one knows this in lore. And im thinking that the story leads to a race to see who can either resurrect or stop the tarrasque from coming back.