How to access:
You need to level up the summon to level 20 to unlock World of Illusions: Diabolos Ultimate, and then farm materials on it to reach level 30 and access the CHAOS stage.
Stage Difficulty: Level 180 (CHAOS)
Mission Requirements:
Score 630k | Clear without any KOs | Clear within 100 turns | Take 15000 or less HP damage
Featured Characters: Aerith, Vaan, Wakka, Sabin, Kuja, Deuce
Various team options with and without boosted characters are available in this event’s Call to Arms!
Fight Overview
This is a single-wave fight with only one enemy, so characters focused on single-target damage are preferred. The fight itself is rather easy, as long as you have a good way to shave down high BRV amounts and bring enough damage to finish it within the turn count. It also has very predictable high-danger phases, good for characters with skills to reduce incoming damage like Porom, Beatrix, Edge or Hope. You definitely want some form of healing to deal with Diabolos’ HP attacks.
Diabolos has double resistance against melee attacks (takes 90% reduced damage), so if you are not bringing Lulu for that combo of 
Enchant/ Resist Down, don’t bother bringing melee units that don’t have a way of getting around it (like Lightning and Onion Knight do, by applying their own element resist down). His other resistances mean no 
attackers (sorry Dark Knight Cecil, you time-traveled too far into the past), Layle having a bad day, and stat-debuffers like Vanille having reduced effectiveness – but DEF Down, Blind and other debilitating debuffs are still very useful here.
Wave 1: Diabolos
| Group Darkga |    attack |
| Shadowblade | Increases BRV when BRV less than INT BRV;  attack |
| Darkwave | Recovers from Break; attack + attack; |
| Diaflare | attack; increases BRV with each hit |
| Shadow Nail |  attack;delays target 1 turn |
| Gravija | Reduces targets’ BRV to 1; increases own BRV; inflicts  3T Turn Rate Down;high turn rate |
Dark Messenger  | Recast ability; increases BRV when BRV less than INT BRV; attack + attack;applies Dark Space to field (increases HP damage dealt by Diabolos); grants  3T Power of Darkness (raises ATK) to self |
Dark powers heightened…!: Raises ATK, DEF, SPD, INT BRV, MAX BRV (activates at 80% HP)
Dark powers further heightened…!: Raises ATK, DEF, SPD, INT BRV, MAX BRV further (activates at 50% HP)
Diabolos is extremely predictable. His first three actions will always be Diaflare, Gravija and Dark Messenger, in that order. From that point on, his actions depend entirely on how full his recast bar is:
Below 75%:
Diabolos will use 3 of the following moves to fill up his bar:
If he is readying an ALL attack, it will be Group Darkga.
If it’s a single target, the move will be Shadowblade, Darkwave, Diaflare or Shadow Nail.
The most dangerous of these is Darkwave, which has the risk of causing upwards of 18k damage if it breaks a character after Diabolos is under 50% HP. Make sure you don’t get broken by that point. Also watch out for the 1 turn delay from Shadow Nail.
At 75%:
He will use Gravija to reduce all your party members’ BRV to 1, inflict 3T Turn Rate Down on them, and gain a huge amount of BRV to himself. This can leave him with up to 40k BRV in the latter half of the fight, and it will happen even if your party has no BRV to begin with (it’s a BRV gain, not a BRV drain). It is a high turn rate move, so there is a risk that Diabolos’ next move might occur immediately after it, leaving you no time to respond.
At 100%:
After Gravija, Diabolos will always have a full recast bar and follow up with Dark Messenger. It’s a group BRV+HP attack and, if you did not shave the BRV he gained from Gravija, this move will dump it all as split damage to the party. Yes, that means it’s a potential 40k split among 3 targets; but the good news is, since this is a World of Illusions fight, your whole team is boosted and probably has more than 15k HP each, so it is survivable.
After Dark Messenger, Diabolos will just repeat the pattern, always using 3 attacks from the first list before the Gravija+Dark Messenger combo. If you have good enough damage mitigation or healing, it is easy to just let him do this thing and heal up after the big attack. But with some decent BRV shaving or a good delayer to stop Diabolos from acting at all, this fight will offer very little risk, feeling more like a marathon to bring his HP down to zero within the limit of 100 turns.