Warmonger Hellkite

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.

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.

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