Unown Dimension is a location that only shows up during a post-game mission. In order to get to this location, players have to encounter 25 forms of Unown. Oculeus then will be called upon after the player has successfully done so. If the player fails to defeat it, it will reappear again at the Egho Sanctum, in place of the Arkhaos that used to be summoned.
Note that Oculeus will not respawn if the player "accidentally" defeats it at the Egho Sanctum. (Note that "respawning" only means it is available to battle again - shininess, IVs, nature, etc. will not carry over)
