I'm really enjoying this game. It's really got the early Dragon Quest feel just right.
This crash seems to be happening whenever an enemy Peryton tries to do something:
############################################################################################
ERROR in
action number 1
of Step Event0
for object objBattle:
DoAdd :1: undefined value
at gml_Script_anon_gml_Object_objBattle_Create_0_3473_gml_Object_objBattle_Create_0
############################################################################################
gml_Script_anon_gml_Object_objBattle_Create_0_3473_gml_Object_objBattle_Create_0 (line -1)
gml_Object_objBattle_Step_0