Ghost Portal was a long time datamined skin and was originally thought to be paired with the Cloak Star skin. It turns out that it's part of the Skull Trooper Challenges and can be only obtained if you have the Skull Trooper skin and you complete those challenges.
It also has new styles, there's the original, green, and purple. The purple version, however, is limited to players who originally purchase the Skull Trooper during its first release.
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Ghost Portal's Attachments
Attachments are items that come with a cosmetic when a particular one is purchased.