Carnage, Crimson Chaos

You must follow all normal timing rules when casting a spell with mayhem. For example, you can't cast a creature card with mayhem you couldn't normally cast during an opponent's turn, even if you discard that card during an opponent's turn.

If a card in a graveyard has X generic mana in its mana cost, X is 0 when determining its mana value.

If the returned creature can't attack for any reason (such as being tapped or having come under your control that turn), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.