Who could be the main antagonist in a 'Harry Potter meets the Morrigan' fanfiction?

Another possibility is that there is some sort of magical imbalance that only Harry and the Morrigan together can fix. Harry's knowledge of the wizarding world and the Morrigan's ancient and powerful magic combine to face a great evil that threatens both of their worlds. Harry has to learn to trust the Morrigan, even though she is a very different kind of magical being from what he has known.

It could still be Voldemort. Even though the focus is on Harry and Nagini, Voldemort is Nagini's master and the main source of evil in the Harry Potter world. He would likely try to stop Harry from getting too close to Nagini or from finding out any secrets Nagini might hold.

It could be a dark wizard who is jealous of the bond between Harry and Lily. This wizard might try to break their connection and use Lily's power for his own evil purposes. Maybe he wants to control Lily's magic, which is known to be strong, to achieve domination over the wizarding world.

There could be a group of anti - magic fanatics in the fanfiction. They see Byako as an abomination because of his strange powers that don't conform to the normal rules of wizarding magic. They also view the Hogwarts students' acceptance of Byako as a sign of the corruption of magic. So they set out to eliminate Byako and anyone who stands in their way.

It could be a dark fairy who wants to use the power of both the wizarding world and Pixie Hollow for her own evil purposes. She might try to steal the magic from Hogwarts and the pixie dust from Pixie Hollow to become all - powerful.

Maybe the main antagonist could be Ra's al Ghul. He's always interested in power, and if he got his hands on some magic from the Harry Potter universe, he could become an even greater threat. He could try to use the magic to bring about his vision of a 'perfect' world, which would be at odds with both Harry's and Nightwing's ideas of justice. So they'd have to join forces to stop him from achieving his goals.

A dark wizard from the Harry Potter universe could be the main antagonist. Maybe a follower of Voldemort who escaped the final battle and found his way to Byakko's world. He sees the power of Byakko and wants to use it for his own evil purposes. He could try to enslave Byakko and use its power to take over both the wizarding world and Byakko's realm.

Most likely, Medusa would be the main antagonist. Her ability to turn people to stone makes her a very dangerous foe for Harry Potter and the other characters in the wizarding world.

It could be a power - hungry alien leader who wants to take over Hogwarts and use its magic for their own empire.

Maybe the main antagonist is an ancient magical being that has been awakened by the disturbance caused by Harry's exploration of the 'Mind of Magic'. This being believes that the wizarding world has strayed from the true path of magic and sees Harry as a symbol of this deviation. So, it decides to target Harry and try to bring back what it considers the 'pure' form of magic, even if it means destroying the current wizarding society.