The Bitter Sons was a warband of the Iron Warriors Traitor Legion. The Chaos Space Marines of the Bitter Sons were deployed to the Macragge System as part of the Ultramar Campaign of Abaddon the Despoiler's 13th Black Crusade in 999.M41. They managed to capture one of the Hive World of Ardium's three subterranean hive cities -- Hive Magmaria -- and fortified it against the Ultramarines' expected counterattack.
However, during this campaign, the Ultramarines Primarch, Roboute Guilliman, was resurrected. He assumed command of the defence of Ultramar and led the Ultramarines' 4th and 6th Companies into the hives of Ardium to drive out the forces of the Heretic Astartes. Guilliman launched a surprise attack with the aid of some of Ardium's own Planetary Defence Forces on the Bitter Sons from Magmaria's under-sump runoff locks and thermic processing stations, which caught the Bitter Sons off-guard. The Iron Warriors force was ultimately destroyed.
The Bitter Sons' Power Armour is the same as that of the Iron Warriors, tending to muted and dark metallics, weathered by the aeons and heavily scarred by the rigours of battle. Much could be said of the souls of those who wear such wargear, for Iron Warriors like the Bitter Sons never forget the inequities that befall them, using them as fuel for the fires of their hatred.
The Bitter Sons warband badge is that of the Iron Warriors, an iron mask formed into a shape of a skull often superimposed over the symbol of the eight-pointed Chaos Star.
- Gathering Storm - Part Three - Rise of the Primarch (7th Edition), pp. 41-47