The secret Discovery Battlestar of the ninth week of Season 8 can be found on the Viking ship in the snow world.
To actually make the battlestar appear, you must first complete a the full set of weekly challenges.
Once any four sets of them have been completed, this secret Battlestar can be collected and the challenge “Find the Secret Battlestar in Loading Screen #9” will appear in the Discovery Challenges. It will reward you with one full battle pass tier.
An overview of all challenges of week 9 in season 8 can be found here.