It's an alternate game mode. About the time you stopped, they introduced a bunch of new game modes over time.
One for All: Everyone plays the same champion on each team.
Nemesis: The enemy team picks your champions for you.
Doombots: Co-op vs. AI against some obscenely hyperbuffed bots.
Ascension: A kill-based brawl in the Dominion map with Ascended Xerath in the center as a semi-epic monster. Whoever kills Xerath gains immense power at the cost of sustainability.
URF (Ultra Rapid Fire): 80% CDR and infinite mana for everyone, on top of a heap of other buffs.
Legend of the Poro King: ARAM map. Like regular ARAM, except you throw snowballs at each other with a one minute cooldown summoner spell. Landing 10 or so snowballs summons the Poro King, which acts as a super minion.
Burning Tides: Special Bilgewater-based event. A whole heap of new items and minions are added into the regular SUmmoner's Rift game for an extremely chaotic experience. Also the announcer is replaced by Gangplank.
Definitely Not Dominion: Like regular Dominion (which was removed earlier this year), but now the minion push is linear and each team has control over one node at all times.