Effective NPC is an 5E extension that enables effects in the custom effects list to be added to the NPC sheet by adding the text "has become customEffectNameTag"
Add a custom effect Engulfed; Restrained; SDMGOS: 6d6 acid
Modify NPC sheet and add "has become engulfed" in the npc action or traits
Add NPC to combat tracker
The engulfed effect can now be applied from the NPC sheet