I'm concerned about something, Titans are huge, and the Temple of Sacred Ashes had a lot of Lyrium veins, if that's part of the Titan we met, then does that mean that it is being corrupted?
Well, if we're going to get into it - and we recall that the temple of Sacred Ashes appears to be a converted Temple of Mythal (the fight with Corypheus reveals that under the ruined floor of the temple a mural of Mythal is hidden) - there's a much more interesting theory: the Evanuris built their temples on the corpse of a Titan they slew.
There is a shot of the temple of Mythal floating around from an airborne POV that shows it built on a crevice that looks a great deal like the altar for the titan in Descent. We get hints in Trespasser that the Evanuris went to war with the Titans, that Mythal slew one, and that the world bloomed.
I think the temples of the Evanuris, the seat of their power, was built on top of the corpse of a Titan. This works with other theories - namely that the "old gods" are the bodies of the Evanuris, their "prison" is actually the temple they once ruled as God-kings, and the beings we think are "old gods" are actually abominations like Hakkon.
If we accept the theory that the elves were once spirits, then the idea of making a new God - like how the Avaar constantly remake their own gods by willing a spirit to be like the god their remember - is not so absurd. It's quite easy to see how the elves trapped in a world with the Veil would long to re-create their old world.
Anyway, back to the Titans. I think we're too stuck on thinking of Thedas as a purely material world. If the Veil is truly a fabrication and the real nature of the world was a lot more malleable - and seeing as how so many of the places the Eluvians connect to are akin to floating masses of land suspended in nothing - it may well be that a Titan is just a being that "grows" on top of one of these masses. Lyrium would just be a vein - it would be holding the world together.