Where do you learn Atramentous Libations?

[Gameesh]

From the latest update notes:

Atramentous Libations (Lv.99 Recast time: 180 sec.)
Steals HP from enemies within range. Ineffective against undead.

Now, I consider myself an experienced adventurer but I can't recall ever seeing this weaponskill on anything.
The wiki's aren't helping much and none of my friends in-game have a clue what monster does this skill.
My best bet is either Dullahan or Flutterini since there's so little information about them.
But if it isn't those two, then I don't know.

Can someone who has learned this spell tell me where he or she got it? Thanks in advance =)

[Tidis]

You learn it from Dullahan mobs.

[Gameesh]

Thank you very much =) Once I've learnt it I shall add this info to the wiki's. Both of them aren't 100% up-to-date anymore these days.

[Olor]

You learn it from Dullahan mobs.

Thanks so much!

[Gameesh]

Learned the spell and added the information to FFXIclopedia.
Added Molting Plumage, Sweeping Gouge and Nectarous Deluge as well!