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What do you mean "I have to?"
If I only removed one of the monsters in the hall in that fashion, only one new monster would repopulate the hall. Could I not still decide to attack one of the pre-existing monsters?
Sorry, thought you were asking what happens if you remove all monsters in the hall with Creeping Death.
Yes, you must attack A monster, but it can be a new one or an old one since Creeping Death happens before you declare your target of battle.