In Break-In At Bleak End, Hallannaë was pulled from the Upper Realms into the Prime Material, and was no longer an ordinary unicorn. Summoned from the chaos of a Wild Magic surge invoked by the Devilborn Sorcerer Mercy, they were marked with unmistakable traces of his Infernal heritage: a blackened coat, an icy-white horn, and a billowing mane of wild energy.
Appearing at the end of a deadly battle in the Murkmire Marshlands, Hallannaë healed the Whistledrop Crew without hesitation, as their kind were wont to do, and gathered themselves to impart something vast. They looked to tell the Mortalborn about horrible planar threats that were set to destroy the Material, but disappeared just before they could impart any advice.