The Warden/Alistair/Loghain killed the Archdemon during The Battle of Denerim.This decision is supposedly recorded in the codex entry "A Study of the Fifth Blight, Vol. Loghain was spared or executed during The Landsmeet.
Note that if you import an Orlesian Warden-Commander created in Awakening, it will warn you upon importing to Dragon Age II that it will fill in the missing details using the pre-built histories.
Note: see Results and pre-built histories for full list of decisions