grudge

You hit a zombie. It runs. You let it go, because it was just a zombie.

It comes back with a name.

Grudge turns any mob that escapes a fight with you into a Rival: it gets a name, a
title, and a grudge against you specifically. It adapts to the way you fight — burn it
and it stops burning, shoot it and it picks up a shield. If it kills you it takes a
piece of your gear and wears it. It hunts you down on its own schedule, and it comes
back even if the chunk it left in unloaded hours ago.

Kill you enough times and it ascends: held in place for six seconds, convulsing,
bleeding, growing, veins lighting up under its skin, until it roars and the shockwave
puts you on your back. Then it comes for you at twice its size with three times the
health — and if it was a pig, it now has teeth.

Kill it and you get a trophy engraved with everything it did to you.

No dependencies. Server-side logic. Works on top of any modpack, with mobs from any mod.