Dungeons & Dragons Death Tyrant

My son recently got into D&D and was setting up a One Shot as the Dungeon Master. He wanted a particular monster to fight and after a brief search found MagicEngineer's Beholder https://www.thingiverse.com/thing:1353092 . The Death Tyrant (from what I'm told...) is a nasty version of the beholder that arises after it dies - so it's just a skull. Apparently it's really pissed off for having died and is very strong and vicious....