Waking Dreams  

Summary

Diabolos has informed Kerutoto that he is waiting for you at the Shrouded Maw in Pso'Xja, buried deep in the northern lands. You must respond to his summons so that Kerutoto can meditate free of trouble.

Walkthrough

First of all, to get this quest, you have to have completed Chains of Promathia mission 3-5, Darkness Named. There is no fame requirement. Head to the Rhinostery and talk to Kerutoto to get the quest; she'll mention that Diabolos has been pestering her in her dreams and it is keeping her from being able to meditate properly. She'll give you a key item: Vial of Dream Incense, which is basically your tuning fork for Diabolos. Similar to Tuning Forks and Moon Baubles, everyone in your party must have one to enter the battlefield.

To get to Diabolos, you'll have to go through the level 40 capped tower of Pso'Xja, located at (H-8) in Beaucedine Glacier. Since you are capped at 40 and there are mobs that aggro sight, sound, and magic along the way, you'll need Silent Oils and Prism Powders to get through here. If you don't remember the way to The Shrouded Maw, the order of warps you must enter is Red -> Black -> Red -> Black. Going through a warp drops Sneak and Invisible, so make sure to re-apply them after each warp. After the last warp, you'll come upon an elevator, which takes you down to the basement. Go through the Stone Door here to enter The Shrouded Maw. You'll be uncapped here and can prepare for the fight against Diabolos Prime.

Once you defeat Diabolos Prime, you will receive the key item: Whisper of Dreams. Head back to Kerutoto for a cutscene where you can choose your reward: the ability to summon Diabolos, a Diabolos's Earring, a Diabolos's Pole, a Diabolos's Ring, or 10,000 gil. Like other avatar quests, this quest is repeatable once per real-life day.

Battle Notes

Diabolos Prime is similar to the mission fight, except that this time the fight is uncapped, so he has more moves, including his 2-hour: Ruinous Omen. This attack will drop your whole party's HP by either 25%, 50%, or 75%. It's completely random and there is no way to defend against it, so you basically have to rely on luck.

He still removes tiles at half health, though the tiles he removes are apparently different this time. Unfortunately, I don't have info on which tiles he does drop for this fight. The common strategy for this fight seems to be BLM cannon PTs, with a 74+ RDM/DRK to Chainspell+Stun him, but obviously it can be done with a more reasonable setup as well. Diabolos Prime is on par with Fenrir Prime in terms of difficulty, though a party of characters lower than 70 has little to no chance of winning.

Spoilers