The only thing I can think of to explain it is he wanted you to fetch the components needed to access the Hall of Echoes, but the biggest hurdle (the Patriarch) is accomplished when he ambushes you in the Primordial Cave after the Hall of Echoes is opened. Ba'al, a demon general of his, has the sigil. With your death would come the Battle Tower, and thus the spell. Zandalor in Aleroth has the Shield, but as we see in FoV, Aleroth is hardly in the best position to be resisitng attempts to claim the shield.
Maybe Damian was waiting until the Dragon Knight reached his/her full power, and then turn him/her to his cause.
Is it even certain that Damian knew what Maxos knew? I.e. the spell, sigil and shield? It appears not, otherwise he would certainly have appeared when the Dragon Knight opened the Hall of Echoes and finished him/her off at last.
