MagisterCultuum
Great Sage
Living units return as Mercurian Angels of they have a God religion OR were Mercurian units in life. There is one summon in the game that is a living unit: the Mistform. This makes Shadow III an awesome spell in Mercurian hands, as it would allow you to kill units of good religions to claim their souls without declaring war, and if your summon instead dies you still get a free angel. Shadow III can pretty much guarantee an extra angel per turn.