The Unicorn is an innately magical creature, but its selfish nature means that it tends to feel no kinship with creatures that were summoned or created through sorcerous means, despite their common origins. If anything, it pities such beings for their misfortune at having been created as anything other than a Unicorn. Unicorn ivory is a much sought after prize in certain corners of Bretonnian society, and many a gallant knight has met his end pursuing a Unicorn deep into Athel Loren.
Just as the brave warrior thinks he has cornered his prize, the creature disappears without warning, conveniently within a few paces of a swarm of vengeful spites or vigilant Waywatchers. Curiously, Unicorns are drawn to female mages as moths to a flame, and find the taste of magic intoxicating. Most Spellsingers and Grail Damsels find this an acceptable situation, as a tame Unicorn makes for an excellent steed. Furthermore, the beast’s nature protects its rider against hostile magics, with a devastating spell often resulting in little more than a slightly inebriated and emboldened steed.
The Unicorn is a deceptively powerful mount choice for your spellcasters. First and foresmost, it provides 40% Magic resistance which is certainly notable if you know your opponent will bring some magic attacks or rely on direct damage spells to hurt your caster (Damsels also have an optional ability that provides 12% magic resistance in an aura around them, further reducing magical damage)
Secondly, it provides a solid melee attack boost (+6 or +7 depending on the unit). While, you don't want spellcasters to be doing fighting, they are useful for charging and flanking an enemy from behind. Improving their melee attack improves their ability to flank and deal damage overall.
Thirdly, there is a sizable charge boost from the unicorn compared to the warhorse or elven steed, which again improves their damage output on the charge.
Damsels specifically go from 76 speed on their warhorse to 90, making them much more nimble in combat.