Reaper

''"The Reaper is a powerful, relentless spirit, who chases the dawdling and idling." STR +7 AGI +7 HP 999/999 DAM''

The Reaper appears shortly after Turn 350, at which point you should have finished the level already. On the turn it spawns, it does not affect the numbers on tiles, which is most likely a bug. It is not vulnerable to the Resealant.