Also known as League of Legends: Arathi Basin. It is live, a map with 5 capture points instead of three lanes. The overview has information about the new mode. I haven’t tried it myself, but the games should be shorter (~20 minutes).
Of course, quite a few League of Legends games are decided at the 20-minute point, because that is when a team can forfeit, but that’s another story.
Update, having tried it: Wow, that is a MUCH faster game. Team battles start within 90 seconds. I have been ahead 200 (of 500) and lost and behind 200 and won. The map is much smaller, the minions are fewer, and everything just feels very INTENSE NOW GO GO POWER! versus the stately pace of the League of Legends early game with its laning and harassing.