Light's Vengeance was the hammer used by Arthas Menethil during his life as a paladin.
This war hammer was used by Arthas before he became a death knight and eventually the Lich King.Taking up Frostmourne, Arthas threw Light's Vengeance away.The hammer is shown to still be lying in Frostmourne Cavern in the Dragonblight ever since Arthas discarded it for Frostmourne.
Players can see how the hammer is discarded in a flashback scene for  Frostmourne Cavern. Darion Mograine also wants adventurers to collect the hammer for the quest  The Sacred and the Corrupt.
It shares its model with the [Grand Marshal's Battle Hammer], and does not appear as it did in Warcraft III.
It is the future Lich King's doings that the players can obtain the hammer and use it to craft Shadowmourne, to defeat him.