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What happened on Arthas Moloch? Who was the unknown enemy that was engaged? Is it in in any of the other codexes or books?

Only that the structures and artifacts found there match none of any known faction, either Human or Xenos; potentially C'tan/Necrontyr, or even Old One, we don't know.

Most people I believe think it was early Necron maybe Old One tech. It might even be chaos, not really sure. Now I think it was some power weapon from the Old Ones, and then the planet was lost when the Necrons went harvesting.

Tun_Tau wrote: Is it in in any of the other codexes or books?

Nothing to find there.
A common take on the event is based on the model, specifically the weapons design.
As it looks like a "kopesh" and the 'ancient culture / egypt styled gear design ' has been used for Necrons, the usual suspect is guessed from a design that may or may not have any hint who created that weapon. The attacker and the creator of that weapon must not be identical..

Artifact worlds are worlds which were home to long lost civilizations for example and thus anything found there could originate from very far in the past and the name of the creator(s) lost in time ( or precisely GW didn't deem it neccessary to supply one ).
Secondly the opponent on such world is possibly another group interested in the valuables there or those who defend these places but never leave. The first ones don't have to come from a species known to us, the second ones could be unknown in the current 40k galaxy because some are forgotten or rather automated defense systems that wouldn't communicate or in a dead language so even Farsight himself couldn't identify them.

Plus it wouldn't be a mystery if GW told you... and that "rumor of unknown origin" was the goal from start.