At various points you may have encountered evidence of Grutch.
You never actually interact with Grutch, but you do affect their existence. Grutch is a skeleton you observe being summoned by the cultists in Reboot Hill. If you allow the summoning to complete (rather than killing the cultists yourself), Grutch takes revenge on various necromancers, wiping out their summoners, and killing the lone, sleeping cultist in Shroomcave. You can also find the name scribbled into the table in the Buffalo Pile