The Neo-Soviet marine biomods
Classified as a category 4 threat, the Elib is somewhere between a vampire lord and Godzilla for limits of escalation. Yes, I would consider using a nuclear device to kill just one. I am that sure that I want it dead.
A group of adventurers come across a child’s body on their adventures, odd being such a remote location. Their is no detectable sign of violence to the child, nor are their any signs of life. Suddenly the child opens its eyes, looks towards the group and introduces themselves to the adventures.
If this wasn’t odd enough, the child can’t remember where they are from, only their name and age. Stranger yet, the child has a tattoo on their right shoulder of a family crest, to a family that died out over 200 years ago.