After defeating Phrekyos, the player then has the option to ask him a keyword. To fight Deumion the player must choose the word "Destruction." After speaking about this, the party will have to face Deumion in battle.
He's the only enemy in the game that doesn't have a bestiary entry.
Each of Deumion's attacks (Blizzard, Dispel, Blaze, Holy, Flare and Starfall) are at level XVI. Deumion seems partial to Dispel and Blizzard. Deumion's ultimate attack, Starfall, can be devastating if the party is not protected as it can easily do a couple thousand HP in damage.
When the party defeats him, they will be granted access to the Destroy Tome. This is the most powerful spell in the game, even more so than the fabled Ultima.
To get an advantage over Deumion the party should have Haste, Blink, Shell, and Protect above level 10. To make this battle easier, the player should try to distribute the spells throughout the party, rather than one character. When the battle begins, the party should cast Haste, Shell, Blink, and Protect on all characters. In the second round the two strongest fighters should attack with physical strikes while the others act as medics. To increase their attacks, it is a good idea to cast Berserk on the physical attackers. If Deumion connects with Dispel, the party should redistribute its protections.