One overpowered Fallout 4 baddie versus everything else

The latest Fallout 4 NPC battle could be considered a spoiler. You've been warned. Fallout 4 brings back a particular creature from Fallout 3. I sort of don't want to say who, because if you're a big Fallout 3 fan who hasn't got to that part of the game yet, it may spoil your experience.