I just posted this in the Summoner forums, enjoy:
Well I've just recovered the information as to obtaining the brand new avatar, Diablos.
There are some prereqs.:
You must have beaten Diablos in the CoP mission "Darkness Named".
You must have completed the quest that Kerototo gives you in the Rhinostry in Windurst Waters.
_
Once those steps are taken, and you talk to Kerototo, he will give you a key item, "Vial of Dream Incense". Take this item to The Shrowded Maw, and you will initiate your Diablos battle.
Short guide, eh? Well that's all you need to know!
Happy hunting Summoners!
-Neejee