How do you get the Prestige Edition Dark Cosmic Diana Icon?
It came with a Prestige skin. Riot's description in the client reads: “This icon was acquired by earning Prestige Edition Dark Cosmic Diana during the Worlds 2024 event.”
Is the Prestige Edition Dark Cosmic Diana Icon still available?
Only with its Prestige skin. The Prestige Edition Dark Cosmic Diana Icon comes with that skin, so owning the skin gets you the icon.
