Did the demon king play a part in the war?

Just a few things that are unclear to me. What role did DK play in the war? We know he forms a brief alliance with the goddess clan rule at some point to oppose Mel and El but was this after or before the war? How did he lose his power and why didn't he intervene when the TCM were sealed? And what about the power he gave to Zel? Further, if Zel was as powerful as the demon king, how did the TCM get sealed in the first place?