Ancient Devil

The Ancient Devil is encountered in Otka Island, in the unmapped dungeon section that can be accessed after the player has obtained the Sol Blade. The Ancient Devil guards the Catastrophe Summon Tablet.

As a boss
The Ancient Devil has 11783 Hit Points, 500 Psynergy Points, 483 Attack, 220 Defense, 330 Agility and 50 Luck. Concerning elemental attributes, it has a Venus resistance of 180, Mercury resistance of 130, Mars resistance of 150 and Jupiter resistance of 72 and uses its available Venus based attacks with a Venus power rating of 110 and its Mars based attacks with a Mars power of 105.

The Ancient Devil has access to the following moves:
 * Demon Sign: This ability allows the Ancient Devil to take control of one of the player's Adepts. If the Ancient Devil is felled, so too will this Adept. If the Adept is felled, it will rejoin the party in KO'ed condition. The Ancient Devil will only use this move to take control of one Adept at a time.
 * While under the Ancient Devil's control, the adept is often ordered to attack, occasionally use Psynergy, and is capable of using your summons against you. If the Adept has any healing items in his or her inventory, he or she will use them on the boss, but never themselves.
 * Haures: A summon sequence that calls the blue-skinned demon with red nails and fangs to wreak havoc upon the party. This deals a Venus-aligned attack with a Venus base power of 270 with additional damage equal to 15% of the victim's maximum HP meter. All targets also have a chance to get inflicted with the Deadly Poison condition.
 * Dire Inferno: A Psynergy spell that conjures a massive ornate shape to drop a big fireball, resulting in a huge explosion. It deals a Mars-aligned attack with a Mars power rating of 200 and a range of 5 targets. This consumes 32 of its user's PP.
 * Undead Might: A Psynergy spell summons a huge skeleton warrior to hit the target with powerful force. It deals a Venus-aligned attack with a Venus power rating of 250 to one target and consumes 30 PP upon usage.
 * Curse: This Venus Psynergy may inflict the Death Curse condition at one target and consumes 6 of its user's PP.
 * Psy Drain: A Psynergy spell that attempts to steal one target's PP with a Jupiter power rating of 20. Consumes no PP.
 * Attack: The Ancient Devil's physical attack where it uses some magical force of its book to deal damage by shooting the target.

Felling the Ancient Devil nets the party 55216 experience and 3438 coins and the summon tablet it was guarding becomes approachable.

Strategy
The key quirk to this fight is that one of your own Adepts will be fighting against you almost the entire fight. The possessed Adept will usually just attack, though will sometimes use some of his or her own Psynergy and if he/she has some healing items in their inventory, they will often use those on the boss. However, the most dangerous aspect of the possessed Adept is that they can (and will) use your summons against you, drawing from your pool of stand-by Djinn. Furthermore, they will summon with only a couple of Djinn on standby, even if it's an entirely incompatible summon (there is nothing stopping Angel Rief from summoning Kirin). As such, if you do plan on summoning, plan on unleashing no more than a couple Djinn at a time, and summon that summon in the very same turn as the second Djinni being unleashed, otherwise you run the risk of the summon being used against you. If you want to open with a powerful volley of summons, place your fastest characters in the front line and put enough djinn on stand by in such a way that they can all be used in a single turn.

There is nothing stopping the boss from possessing lethal characters, such as Savage Fiend Sveta (which you JUST put into Beastform), or Master Matthew with mastered Sol Blade and 99% unleash chance; or highly useful characters like Pure Mage Rief. As such, have more expendable characters in the front line and switch in such key assets after the first round, as the boss will always use Demon Sign as soon as he can if he does not have a possessed Adept (this includes his very first move).

Throughout the fight, focus your offense on the Boss. The possessed Adept will go down either to the area of effect of your attacks or will go down once the boss is dead. Since the boss will always possess a new Adept as soon as the current possessed one goes down (putting the now KO'd Adept in the back line), you should try to avoid taking out possessed ally unless he or she is going to wreak havoc on you (such as one of the aforementioned examples). With the exception of being able to summon Haures at will, he is not the most damaging of bosses, and a character or two constantly casting Wish should be more than adequate to keep your party alive. Try to boost your resistances and defenses to mitigate Undead Might and Dire Inferno (his Attack shouldn't be that much of a threat). The fight can be a long one (as it might be difficult to pull off the more powerful summons), but isn't overtly difficult.

Trivia

 * The Ancient Devil bears a striking resemblance to Baphomet.