Addition:
The challenge to “Slay a Blood-Moon Beast” has a picture of the Beast in the Fractured Citadel, on the Isle of Siptah. But slaying the Beast in the Fractured Citadel does not count towards the challenge.
We know there are several NPCs with the same, or very similar name. It might be worth checking that the challenge counts them all, because otherwise it may be very difficult to find one that counts for the challenge. I haven’t had an opportunity to test the one spawned at the Pools of the Grey Ones or the one spawning during the Maelstrom yet, but it would be great if the one that is pictured in the challenge actually counted towards the challenge.