Thwack is an instant-death spell that targets a whole group of enemies, stopping the heart (or nearest approximate). Typically, characters that take up the paths of Priests and Sages are the only humans that learn the whack line, giving the impression of the spell family being a case of "forbidden" magic reserved only for the most pious and disciplined.
Dragon Quest II: Luminaries of the Legendary Line
Thwack is learned by the Prince of Cannock at level 23, and costs 4MP to cast. Infamously, frostburns will cast it ceaselessly in the NES version.
Dragon Quest III: The Seeds of Salvation
Priests and sages will learn Thwack at level 28, now costs 7 MP to cast remaining so until IX.
Dragon Quest IV: Chapters of the Chosen
Kiryl learns Thwack at level 24 and costs to cast.
Dragon Quest V: Hand of the Heavenly Bride
Dragon Quest VI: Realms of Revelation
Thwack is learned by advancing to rank 7 of the Priest vocation.
Dragon Quest VII: Fragments of the Forgotten Past
Thwack is learned by reaching the first rank in the sage vocation in the original and 3DS remake.
Dragon Quest VIII: Journey of the Cursed King
Angelo learns Thwack at level 22.
Dragon Quest IX: Sentinels of the Starry Skies
Thwack is learned by Priests at level 34, and now costs 10MP. It works off of the priest's Magical mending, raising the base accuracy from 50% once the stat hits 100 points, capping at 100% effectiveness (barring resistance) at 999.
Dragon Quest X
Thwack is a spell exclusively used by monsters, and it costs 10 MP.
Dragon Quest XI: Echoes of an Elusive Age
Serena and Rab learn Thwack at level 49. It costs 10 MP to use. It has a 50% success rate barring resistance, and its success is based on the user's Magical might.
Dragon Quest Monsters
Called Defeat in this game, it is an upgraded form of Beat. Beat grows into Defeat when the monster reaches level 24 with at least 80 MP and 98 intelligence.
Dragon Quest Monsters 2
Defeat now requires 177 intelligence to learn.
Dragon Quest Monsters: Joker
Thwack can be learned from:
Dragon Quest Monsters: Joker 2
Thwack can be learned from all the skills it could be previously, as well as from Exorsus for 45 points, or Mortella for 85 points.
Super Smash Bros. Ultimate
Thwack is a spell the Hero can randomly choose from his Command Selection ability. It costs an immense 30 MP to cast and will only deal 3% upon failure, but creates a large explosion with a chance to instantly KO an opponent. The chance to KO increases with the opponents damage and is slightly boosted if the Hero is also damaged.