The main issue with Tumeken being a child of Mah would be explaining how he got off the plane in the first place.
He didn't kill a god (unless Seren and Zaros have a third sibling they don't know about).
The Rituals aren't a viable method for ascending (at least not on Freneskae) as we saw with the Chelon Mah.
It could be an elder artifact, I guess (which would explain the still unaccounted for artifact in the Desert), but I personally question the magnitude of such an artifact's power given the potency of the Measure.
Maybe he left the plane via the World Gate some time after Guthix had visited, and then found his artifact and ascended (not necessarily in that order) afterwards.
22-Nov-2016 15:56:48