Legend Of The Poro King takes place on The Howling Abyss single lane map (as you'd expect given that's the poros' natural habitat). The twist is that each player now has two poro accompanying them into battle. Poro – in case you were wondering – are those tribble-y looking balls of fluff and horns bounding about.
What these poro do is give the player a couple of Lee Sin-adjacent abilities. First there's Poro Toss where you hurl the smaller poro across the Abyss. If you actually hit someone you can use the ability a second time to dash to them. So it's like Sonic Wave/Resonating Strike but a bit fluffier. If you land ten of them your team summons the Poro King – a supersized minion which deals AoE damage, regens mana and heals your allies.
When he's roaming about you can use another poro ability called To The King! to launch yourself and land by his side.
As per Riot:
Keep in mind, summoning the Poro King is a massive boost to your team’s sieging potential. Don’t underestimate the AoE damage he lays down and the AoE heal he serves up. Pitch a poro every time Poro Toss is off cooldown, because the first team to rack up ten poro smacks receives a royal visit. When the King’s on your side of the fight, protect him. Conversely, depose the King immediately if he’s leading the other team.
In case you've been furrowing your brow and screaming "BUT HOW DO I PORO SUCCESSFULLY, PIP? Why won't you tell me???" while reading this post, calm your beans. I can go one better and show you via this here video of the professional showmatch for the mode which played out yesterday featuring members of Cloud9, Curse and Team 8:
Legend Of The Poro King will be sticking around til January 6th, 2015.
(A word of warning if you're new and think this looks like a laugh, though – access is gated to summoner level 10. That doesn't take long to achieve, I just thought I'd let you know access might not be instant if you're only a fledling LoL player!)