Ithuno has a 20% chance of spawning for every entry into the area. Upon entering Special Op Sector 5, Ithuno will be in a treasure chest near the end of the area, behind a big rock on the train tracks (also near one of the Surianders and after the Adamantitan). If a chest appears there, one will know it is Ithuno if the bottom section of it is purple and the chest itself is silver (other chests in the area are green).
The Deathbringer can be stolen from it. It's possible to get more than one if the player keeps spawning Ithuno.