Hideout of the Lost Legion is a small dungeon that can be found in Battlefields of Eternity in Diablo III. Inside, the remnants of the Lost Legion of the Fallen are stationed.
To defeat them, players need to kill each Fallen in cave, including Shamans. This will spawn the three champions: Hakkajar, M'Dukkan and Rejakka. Killing them will cause a tide of 100 more Fallen to (literally) fall upon the heroes, and killing them will finish the event.