The Lion's Roar Within Naraku

Orthus is located in the 2nd Stratum, in front of the small room southeast of the stairs. (Refer to the map below) It will be stationary so you have enough time to prepare and heal up before engaging it. Make sure that you or at least one of your Demons can use Bufu.

Orthus LV8

Fire: Drain

Ice: Weak

Dark: Resist

This is a special demon that you must bring down to complete the Quest. Orthus has two Press Turn Icons which allows it to attack twice in a turn. It's Bouncing Claw attack can land 1-3 hits on a single target. It's Fire Breath attack also damages multiple targets so prepare to heal up as necessary. If you or your Demons are near Orthus' level then you shouldn't have too much trouble taking it down.