Yeah, you check when the enemy is dead, and set a switch for that monster. Every turn, if that switch is on you would add 1 to a counter - after the counter variable reaches your limit, you would remove the Death condition and restore HP.

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Well, you can set up a Transform skill for those monsters, but that's not too useful for what you're trying to do.

Another way is to have a living copy of all ZOMBIE monsters within the M. Groups tab, set up as Invisible (right click on the monster). Now, when [xxxx:MakeZOMBIEAlive] is set, all normal monsters come into play, and all ZOMBIE characters die off. This will of course have to be tailored to every specific enemy group, which is a bit of work.

The best way to set that [xxxx:MakeZOMBIEAlive] switch? I'd say use an item, something relatively rare or expensive, that would set that switch. You could use a skill, but it might be too easy to "revive" the ZOMBIES.