Necromancer – “Ruler of Erathia” [Campaign 3, Mission 5]

Could you please tell me how to summon a phoenix? When it's on autopilot, the hero summons it, but I can't seem to do it myself. Actually, I managed to defeat Godric using the elemental summoning spell, with an army of 200 archers, 150 zombies, 25 vampires, and 20 liches.