From Dragon Quest Wiki
Blightsplitter is a sword skill in Dragon Quest XI, being the darkness-themed equivalent to Gigaslash.
Dragon Quest XI: Echoes of an Elusive Age
Blightsplitter is learned by speaking to the elder of the watchers in Havens Above in the third act of the game. The Sword of Light or its upgrades are required to perform the technique, and when used it will deal up to 210~250 damage for 16 MP. If the player has completed quest #60 then the MP cost will double, raising the base damage to 200~400 and limit to 400~400.