'Pure of heart' can mean pure evil...
I've posted pictures of Knightmare before, but he's not the only unicorn to have been destroyed by the DemonQueen's evil.
Other dire unicorns stalk the land, each accursed in a unique way. Shown here we have Aerlrys, a creature of purity and life warped into a monster that brings death wherever she goes. Where her bleeding hooves touch the land plants whither and nothing ever grows again. Her eyes are empty sockets and blood runs from every orifice. Aerlrys's horn still contains magic, but only to cause harm for shes endures unending pain and wishes to share it. Since she's blind she uses it to 'see' by detecting the lifeforce of all around her.
Like normal unicorns she can call out to the minds of others but she sings a siren song, its sad, haunting but beautiful melodies entrance those who hear, leading them from safety and to their death for Aerlrys feeds upon living flesh, preferably that of the innocent.