Warmonger Hellkite

If there are multiple combat phases in a turn, creatures must attack during each combat phase in which they're able to.

If, during a player's declare attackers step, a creature is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under that player's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.

Only creatures that are attacking when the last ability resolves will get +1/+0. That is, activating that ability before attackers have been declared won't give the bonus to any creatures, including ones that attack later in the turn.