Giving them Grey Warden blood.
If he wasn't a Magister, maybe he was just an Emissary dying on the battlefield when a Grey Warden accidentally dropped some blood into his open mouth?
(I'm just kidding. He's a Magister, along with Alphonse and Coryflora.
)
I mean, there's probably more to the ritual. The Warden Joining requires lyrium and mages messing with the Fade, which is all kinds of brow-raising. Especially since they used some old Arlathan knowledge in designing the ritual.
Also jungle tribal cannibalism. You use what you got, I guess.
But I love the thought of the Architect reenacting the Joining for Urthemiel.
"Join us, brothers and sisters,"
"RAAAWWR!"
"in the shadows where we stand vigilant,"
*sound of fiery explosions*
"Fine. FINE! You are now a Grey Warden! *chucks the bottle at him and runs*"
If only he'd been able to pull a Duncan v Jory for that particular failed recruit...
Hmm. I wonder what would happen if a darkspawn slew an Archdemon. Presumably, it would pop into an nearby vessel. This could help them make a quick escape from those crazy Warden guys.
But what if it was one of the Seven? Presuming they still have their souls? What on earth would happen then?
Who would be in control? It was implied in Legacy that the Seven were wandering around, Awakening style, leading small armies of darkspawn. But they seem to cool their heels between Blights, and we've only known of the Architect being active in recent years. What gives?