REKKR! It started as a standalone TC for Doom 1, making all sorts of new enemies, weapons, and levels, all of which could be played on vanilla Doom 1. Technically impressive, polished, overall very fun, the Steam edition adds new levels with even more enemies! store.steampowered.com/app/1715690/...

REKKR: Sunken Land on Steam

A retro FPS set in a faux norse-celtic fantasy world in the midst of a war with the dead. Grab an axe, get revenge.