"Haughty and elusive, the unicorn believes it is the grandest creature ever created, and thus it feels responsible for the lives of those poor lesser beings of the forest."Summon Unicorn is a fourth-level Nature magic spell in Heroes of Might and Magic IV that costs eight spell points and summons a number of unicorns based on the level of the caster.
Not much to this spell, if you want a decent number of good melee creatures then go for it, though griffins are probably a more versatile creature to summon if possible. Buffing your existing stacks is probably a better idea early on in the combat.
Though Griffins are more versatile, Unicorns are slightly cheaper, and thus, they will be able to summon more Unicorns than they would Griffins. Therefore, if your hero happens to have both spells, check the numbers before deciding which to cast, unless other factors make one spell preferable to the other.