I used
IsLastHostileActor = 1
as a condition on a spell with only a message box on it :
Event OnEffectStart(Actor akTarget, Actor akCaster)
debug.messagebox("working")
endevent
However I saw this message box pop up for all the actors in the cell. AND I was not in combat at all.
Then I added an Isincombat = 1 condition and only saw the message when I got into a fight with a single actor in a cell actor.
Then I set the condition as so:
IsLastHostileActor = 0
Just to see if the condition was coded backwards but got no pop up message at all.
also tried IsLastHostileActor > 0
I have tested this enough now (last 2 hours) for me to be convinced the condition is not working as described in the wiki.