Card Text: Draw 2 cards, then banish 1 DARK monster from your hand, or, if you do not have any in your hand, send your entire hand to the GY.
vs
Card Text: Neither player can banish cards.
The Verdict
Case 1: Player-A activates Allure of Darkness first, Player-B then activates Imperial Iron Wall in response: Allure of Darkness will still be able to complete its effect, plus Player-A does not need to banish their dark monster (they still show it to the other player to prove that they do not need to discard their entire hand).
WINNER = Allure of Darkness
Case 2: Player-B first activates Imperial Iron Wall before Player-A activates their Allure of Darkness: Allure of Darkness will not be able to be activated in the first place.
WINNER = Imperial Iron Wall
Further Resources
See the OCG rulings here here.