Still havent got my mystery skin

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my friend said that riot never actually said they would be giving a mystery skin if someone owns all champs

http://na.leagueoflegends.com/en/page/features/team-up-pool-party right there

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my friend said that riot never actually said they would be giving a mystery skin if someone owns all champs

Then your friend is a liar.

http://boards.na.leagueoflegends.com/en/c/help-support/3VhOeAbf-pool-party-rewards-distribution-on-going

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• Any account level is eligible
• You need to have registered your Facebook account to the Friend Discovery feature in your Friends List by June 28th 23:59 PDT, and remain registered for 24 hours. You did not need to have added any friends using this feature to be eligible.
• If you have less than 10 champions left un-purchased and therefore are ineligible for a Mystery Champion, then you will be awarded a Mystery Skin instead
• This reward is still currently being distributed.

This is also correct ^

For the mystery champion the only requirement is that you link facebook. Even if you've received 14 day suspensions or lots of leaverbuser penalties you should still get the champion if you're linked. They stated that specifically somewhere as well but I can't be asked to dig it up because it doesn't apply to me anyways.