Riot has revealed that they'll be releasing a K/DA skin for Kai'Sa with patch 8.21. They've also said that skin will have a rarer version, dubbed "Prestige Edition", that can only be obtained with Worlds Tokens.
But what is a "Prestige Edition"? And why did Riot decide to start releasing them? To answer those questions, Riot Games Product Manager "I am Carlos" wrote a post on the League of Legends boards going through the company's thought process, in detail.
He started out explaining the basics of what a "Prestige Edition" is. Here, Riot aims to create a skin that's more a collector's item than anything else. They want the skin to be rare and hard to get but not impossible, as with the Young Ryze skin, to get that "Wow" feeling when you see it during as you're loading into the game. And unlike skin chromas, this is more than a simple color swap. These skins could include things like unique ability VFX, model changes, an exclusive border, and special splash art.
"I am Carlos" then continued this train of thought as he explained why Riot decided to make these special items:
With Prestige Editions we’re striving to capture the the “cool, rare” skin feeling, but without withholding the base skin. Some of us have had the experience of playing a game and seeing the elusive Young Ryze skin in the loading screen. This is one of the rarest skins in the game. It doesn’t have the most features or effects, but everyone knows it’s a rare skin. It’s cool if you have the skin, but if you’re a fan of Ryze that wasn’t able to pre-order the retail collector’s edition, then you’re out of luck. Prestige Editions won’t be this unobtainable, but they are going to be hard to get. This allows players that value rarity/exclusivity an avenue to show off their collection while still giving people who love the thematic the ability to engage with the normal content.
So if you're a fan of Kai'Sa and the K/DA line, how do you get a hold of the rare skin? To start off you will have to buy the Worlds Pass. This will then give you a number of tokens after every match of Worlds. If you want to accelerate your token collecting, there are also ways of gathering them through in-game missions. For example, you will receive, and we're quoting "I am Carlos" here, "a bunch of Worlds Tokens" when winning your first game of the day.