Brief Bug Summary: Netherworld Monks are erroneously still immune to Physical Damage.
More Details:
KMS' Unlimited removed the Physical Damage immunity from Netherworld Monks (Arkarium boss summon). However, the same relative patch in GMS (GMS' Unleashed) did not apply this change. As such, for the last 8 years the boss has had broken mechanics for GMS. Please apply the correct stats for Netherworld Monk so they are no longer immune to Physical Damage.