Unlocks at 5 stars. Will be farmable from the Guild Event Store after he is unlocked at 5 stars.
Requires the following characters, with at least 17,500 power:
Light Side battle (choose 4 with asterisks *)
Dark Side Battle (choose 4 with asterisks *)
This reddit post explains the gear requirements, omegas, & zetas needed to reach 17.5k on each character.