In the time it took for me to attend an esports meeting Kayn, The Shadow Reaper, has made his Rift debut, joining the League of Legends [official site] champion roster and dragging his sentient scythe, Rhaast, with him.
Kayn's whole deal revolves around whether he controls the weapon or the weapon controls him so there's a Lord of the Rings vibe to the story. In terms of how the game gets played this manifests in a unique flexibility when it comes to Kayn's role. As the reveal post notes: "Kayn's the only champ in League who can change his class in the middle of a game."
If you're a fan of character flavour text and lore here's Kayn's snippet:
A peerless practitioner of lethal shadow magic, Shieda Kayn battles to achieve his true destiny—to one day lead the Order of the Shadow into a new era of Ionian supremacy. He audaciously wields the sentient darkin weapon Rhaast, undeterred by its creeping corruption of his body and mind. There are only two possible outcomes: either Kayn bends the weapon to his will... or the malevolent scythe consumes him completely, paving the way for the destruction of all Runeterra.
As you play each game with Kayn, part of the way you build the champion is balancing how much power you give to Rhaast (one of the Darkin - a race in the game - who has been imprisoned within a weapon). In game mechanic terms it's about whether you hunt ranged or melee opponents. If you favour ranged you're a shadow assassin and get the passive bonus dealing extra magic damage at the start of combat. If it's melee your passive heals you for a chunk of the spell damage you deal to other champs.
In terms of the rest of his kit, there's a dash 'n' slash, a damage 'n' slow, a sort of stealth/shadow realm thing where you get a move speed boost and can walk through walls and the ultimate which lets him inhabit the body of a champ he has attacked and then wrench free, dealing damage as he exits.
There are tweaks to these abilities depending on whether you've gone with the Darkin or shadow assassin side of Kayn but generally he sounds ganky, what with the ability to waltz through a wall and mess you up, and also a bit like Dota 2's Lifestealer. Lifestealer's ultimate allows him to infest a unit and then burst out dealing damage. You can't use it on enemy heroes so the implementation of strategy is a bit different but the basic principles are similar.
It sounds like success depends on getting a good start and snowballing from there otherwise you'd just be food from the mid game onwards. Riot are also keen to stress that there are tradeoffs for each form Kayn can take: Darkin has sustain power but it's harder for him to get into the enemy back lines and his crowd control is easier to avoid. Shadow Assassin has no hard crowd control and very little sustain so he's far more of a glass cannon.
Something I wanted to note which is nothing to do with how the champ actually plays is that the key art Riot have put out reminds me a lot of fan-favourite Jinx. Kayn is rocking a swishy long plait and massive weapon and in terms of the expression there's a similar crazed grin. Here are the images so you can take a peek: