Creature â Jackal Warrior
You may exert Champion of Rhonas as it attacks. When you do, you may put a creature card from your hand onto the battlefield. (An exerted creature won't untap during your next untap step.)3/3
You canâ€™t exert a creature unless an effect allows you to do so. Similar effects that â€œtap and freezeâ€ a creature (such as that of Decision Paralysis) donâ€™t exert that creature.
If you gain control of another playerâ€™s creature until end of turn and exert it, it will untap during that playerâ€™s untap step.
If the creature put onto the battlefield has any abilities that trigger when creatures attack or when you exert creatures, those abilities wonâ€™t trigger.
If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exertâ€™s effect preventing it from untapping expires without having done anything.
All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You canâ€™t do so later in combat, and creatures put onto the battlefield attacking canâ€™t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.
All attackers are chosen at once. You canâ€™t attack with Champion of Rhonas, put a creature card onto the battlefield, and then attack with that creature.