Whack is a recurring spell in the Dragon Quest series. It instantly reduces the target's HP to 0, causing instant death. Whack and its advancedforms are in most games only learned by the pious and sagacious, implying that the spell is not common knowledge even among sorcerers.
Whack is learned by priests at level 20, and costs 5MP. The spell is dependent on the caster's Magical mending, meaning that once the stat reaches 50, it will slowly increase the base accuracy of 50% until it reaches 999, where the spell will be 100% accurate barring resistance.
Whack can be learned by Kiryl for 4 skill points and costs 5 MP to cast. Naturally, it is one of his iconic spells. When used, Kiryl will send out a dark incantation that tracks down a random monster and has a small chance to kill them instantly. Kiryl can upgrade the spell and cast Thwack by charging up Whack for a few seconds.
The Hero can cast Whack through his Command Selection ability. The spell costs 10 MP to cast and will send a purple orb forward horizontally. Contact with an opponent will result in either 1% damage being dealt or the foe dying on the spot, depending on a random chance that scales with how high that character's damage percentage is upon connecting with the projectile.
In an interview with Nintendo Power magazine, series creator Yuji Horii noted that the AI in Dragon Quest IV was built such that in its original version, Kiryl's algorithms would always conclude Whack (called Beat in this version) was the character's best offensive option against foes with high HP and the character would end up casting it turn after turn, simply because it "could not get over the power of the spell". The problem was halfway corrected in the balance-adjusted PS1 remake, except that Kiryl almost never casts Whack. A happy medium would be found in the AI for the DS version, carried forward for the cell phone ports.