I want to make a state called "Banish" that emulates the incapacitated state, so i can make a death spell that targets undead. I implemented it semi-successfully with the following -

if state_id == 20 ##the banish state is in slot 20##
state_id = 1
end

This kills the monster when the banish state is added, however, the collapse sequence is not run so the battler image remains on the screen, even though the enemy is dead. Anyone know how to correct this problem?

Are you using any battle scripts that use states? I know there is a problem with some of the SBS scripts that will not allow an enemy to vanish if they are inflicted with a state aside from incapacitated. So you might check that. The other thing you can do is go into your incapacitated state and have it override all other states when inflicted.

why don't you just make a spell that targets undead only? instead of making a seperate death state? or a workaround would be"killing" the enemy when inflicted with the Banish state?
Just to let you know, i would have scripted this, but it is ridiculous since this can be worked around.