When I first thought about this spell I was considering the part in the movie Frozen when Anna is turned to ice, and then considered making it a killing spell – which would be fair given a base level of 25 is very close to the Perdo Corpus instant death of base 30. Instead, an opponent frozen for two minutes is likely to be at the mercy of the caster till they magically unthaw.
Curse of the Frozen Heart
Muto Corpus / Aquam 40, R: Voice, D: Diameter, T: Individual
The target human is turned to solid ice, reverting to normal after the spell expires.
(Base 25, +2 Voice, +1 Diameter)
And a variant for animals, which is far easier due to the different baseline level for turning an animal into an inanimate object.
Curse of the Frozen Hoof
Muto Animal / Aquam 30, R: Voice, D: Diameter, T: Individual
The target animal is turned to solid ice, reverting to normal after the spell expires.
(Base 15, +2 Voice, +1 Diameter)
This and many other new spells for Ars Magica can be found on the Ars Magica New Spell Grimoire.