Cheapest priced copy (Hour of Devastation): $0.35 at Card Kingdom, near mint, their price list as of .
Vizier of the True 
Creature — Human Cleric · 3/2 · Uncommon · Hour of Devastation · #28
You may exert this creature as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, tap target creature an opponent controls.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Hour of Devastation | 28 | Uncommon | $0.35 | $0.49 |
6 rulings from Wizards of the Coast
- 18 April 2017If a creature has a targeted triggered ability that triggers when you exert it, you can exert it even if there isn’t a legal target for that triggered ability.
- 18 April 2017Some cards have abilities that trigger whenever you exert any creature. These abilities trigger when you exert that creature or any other creature you control.
- 18 April 2017You can’t exert a creature unless an effect allows you to do so. Similar effects that “tap and freeze” a creature (such as that of Decision Paralysis) don’t exert that creature.
- 18 April 2017If you gain control of another player’s creature until end of turn and exert it, it will untap during that player’s untap step.
- 18 April 2017All 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 can’t do so later in combat, and creatures put onto the battlefield attacking can’t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.
- 18 April 2017If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert’s effect preventing it from untapping expires without having done anything.