This item sells in the Black Market.
Uses:
Can be sell for 3.5M (3,500,000) at the black market or by getting it by the "Silver Chariot Requiem" boss.
Evolving dark jotaro hat on SP:OH = Demonic Star Platinum For a limited time, you could get this in the "Crates Event" alongside the Shadow Gem.
Currently : Obtainable
Value: Low
Will be unobtainable?: Probably not.