Itâs the last boss from the Jonael Commemorative Orb map, previously she had a different sprite more succubus-like (you can check @Sensate reply to see how she looked like), I always wondered why they changed her sprite to this new one.
I think the ninja-change happened 2-3 months ago if I remember correctly, when they implemented the first nerf patch about rank8 monsters (not this one, the previous one).
Funny enough, this new Diena was more of a pain in the ass to kill than the old one, and even now after the second nerf, sheâs still a tough one for solo-duo players⌠me and my guildmates find ourselves going there quite often to help new members who want to complete the main story no matter what and itâs always funny how hard it is if you blindly try to burst her down (she recovers HP after hitting you with her AoE and drains all your mana with a debuff, I donât know if Dispellers work, you may want to try)