The Black Onyx initially appears alone in battle in the Double Reed Tower of Sand and later in groups of two or three. It also sometimes appears alongside one or two Steam Horns. It is the dark form of Abyss.
Its normal physical attacks are a foot stomp and a peck with its beak with a chance of knockdown. Incinerate is two hits of fire from middle distance on all targets in range, though if Guarded against, it will only hit once. It telegraphs that it is about to use this attack by grooming itself with its beak. Lash Tail is a powerful tail strike on all targets standing directly behind it. Finally, Stifling Blow is six hits on all targets at melee range with a chance of Poison status and knockdown. If, however, all characters are more than a few feet away, then the attack will simply have no effect. It uses this attack at the beginning of a turn and telegraphs it by raising its head and flapping its wings at the end of a turn.
The Black Onyx has some powerful moves, so battling it successfully is all about knowing its attack pattern. While powerful, the moves are generally easily Guarded against if it doesn't catch a target from behind. If it telegraphs that it will use Stifling Blow on the next turn, then move all characters away if it is not likely that they will be able to defeat it before it gets a turn again. At lower levels, it may be easier to lure it to the light instead to make it change into an Abyss.
In Encore Mode, these monsters can present a real challenge, particularly if they catch the party in a back-attack. Have at least one healer in the party and be ready to escape if the party ends up in a bad situation.
- In the PlayStation 3 version of Eternal Sonata, this opponent's HP was decreased by around 15000 points and the amount of EXP granted for defeating it was decreased by just under half.