The Fourth Tyrannic War sometimes called The Great Devouring began during the closing years of the 13th Black Crusade. The Great Devouring was a horrible war for survival to everyone in it and as a result, strange alliances were made.
It has been later theorized by some of the Imperial scholars that it actually saved the Imperium from the 13th Black Crusade when the First Greater Hive Fleet, designated "Bahamut", moved into Segmentum Obscurus and clashed with the Chaos forces around the Eye of Terror.
This war saw the return of the Silent King to the Necrons' power where he united the entire race under his command. Similarly, many Eldar Craftworlds banded together to fight the devouring menace.