Is there a canon chaos space marine group that's ever turned back to the Emperor? Aside from the possibility of the Alpha legion.
>>54210537
Closest I can think of is the Soul Drinkers.
They never turned to Chaos, but they were mutated almost beyond recognition, went renegade and were declared Excommunicate Traitoris, but they always worshipped the Emperor, and they were trying to act in the interests of the Emperor and not the Imperium (hence the excommunication).
>>54210537
Dark Angles
>>54210665
>>54210537
The Shit Drinkers, probably.
Sons of Malice?