From Warhammer 40k - Lexicanum
The Hrud Rising occurred in 980.M35, when the Imperium began a wide scale campaign, that drew in both Space Marines and Astra Militarum alike, to purge the galaxy of a sizable Hrud infestation.
During the campaign, the Dark Angels were tasked with cleansing three Sectors of the Imperium that the Hrud had infested. However the Dark Angels famously only cleared two of their designated Sectors before leaving, with no explanation given, to pursue their own objectives. Their departure earned the Dark Angels the ire of the Ultramarines and resulted in the Ultramarines' Chapter Master filing an official complaint with the High Lords of Terra, though no action against the Dark Angels was taken.
- 1: Codex: Dark Angels (6th Edition) pgs. 36–37
- 2: White Dwarf 49 (2014), pg. 46