It's also just a story - like a Star Trek holodeck episode. Nothing that happens is real.
Anyway, I ran this last week. I gave the corrupted Avatar magic resistance and legendary resistance, but not legendary actions, because: Compared to a regular unicorn CA hits like a freight train, but is squishy. In one round it turned the monk into a wolf and reduced the healer from maximum hp to zero. If it has the unicorn's usual 1LA kick attack it could utterly demolish anything within 5 ft, but the LAs don't allow it to move. In comparison, the shield and the self heal are trivial, doing next to nothing to improve survivability.
If you want to make the CA a real bstd, I suggest giving it these LAs: Misty Step (1), Hoof attack (2), horn ray (3). The horn ray has the wolf transformation effect, but at 60 ft. range.
How the encounter is structured should also be considered. The party is encouraged to rush directly from the fight with the berserkers, which is actually the toughest fight in the adventure, straight to the pool to try and stop the ritual without any recovery time. Then there is a bait-and-switch. The party expect to fight a bunch of werewolves and a priest of Malar, and my lot used considerable resources against them, when they are not the real threat. My players where WTF! when the unicorn attacked them and the (surviving) werewolves ran away.
Overall, this adventure is miss-labelled as level 8 though, it is level 7 at most.