chaos space marine
The Chaos-corrupted version of Adeptus Astartes: they are very similar to their loyalist counterparts, but often characterized by mutations as tentacles, fangs, extra mouths, horns etc., depending on which of the Four Chaos Gods they had pledged allegiance. They often utilize horned helmets, or decorate their power armor with skulls.
See also
This tag implicates adeptus_astartes (learn more).