found this Secret

[Daliniira]

that a lot of skulls needed
https://steamuserimages-a.akamaihd.net/ugc/14175113107121954/43405F3EAE9750DBF0F4911974430C2105098A86/?imw=5000&imh=5000&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

[Mez Koo]

Its not a secret, they're golden skins to spend skulls on when you've got all the upgrades you want.