Its wings beat to the howls of war and fan the fires of rage.

Warmonger Hellkite

Flying
All creatures attack each combat if able.
{1}{R}: Attacking creatures get +1/+0 until end of turn.

Commander Legends: Battle for Baldur's Gate

Commander Anthology Volume II

Commander 2014

Each creature's controller still chooses which player or planeswalker the creature attacks.

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.