Thwack is an instant-death spell that targets a whole group of enemies, unlike Whack (which targets only one enemy).
Thwack is learned by the Prince of Cannock at level 23, and costs 4MP to cast.
Kiryl learns Thwack at level 24 and now costs 7MP to cast, remaining so until IX.
Thwack is learned by advancing to rank 7 of the Priest vocation.
Angelo learns Thwack at level 22.
Thwack is learned by priests at level 34, and 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.