Like the Malfunctioning Mechsuit from Stonevault in Season 1, the mount only drops on Mythic difficulty, which is only available while the dungeon is in rotation. This change is unique to The War Within, though we suspect that all of the dungeons will be in rotation by the time the expansion ends, so players who miss the season won’t be permanently unable to find it.
Unlike the Malfunctioning Mechsuit from Stonevault however, players won’t need to hunt down other dungeon items to complete a quest. This mount drops directly from The Darkness or the Mythic+ chest, still separate from normal loot, and although it’s marked as very rare you may even see multiple awarded in the same group!