The map will have "a richer jungle and more isolated lanes that are difficult to switch between." New neutral creeps will populate the map, which will grant buffs when killed.
Presumably, the new map will be coming along with the announced update to the game's art style, but no release date has been announced. Magma Chamber, if it fulfills its design purpose, should bring a different flavor to 5 versus 5 matches as the reduced mobility will make large scale fights less frequent.
The lore description of Magma Chamber can be found below.
Deep within the obsidian mountain from which the Institute of War is sculpted lies the League's most intense Field of Justice to date - the Magma Chamber. Before the time of the League, the Chamber was the heart of a powerful, but now dormant, volcano. Ancient magma flows cut out a gigantic cavernous room that the League has reshaped for its own purposes. This battle arena was built specifically to address the increasing number of disputes between Demacia and Noxus. It is one of the largest Fields of Justice found anywhere on Valoran.
To that end, teleporting platforms provide champions with the ability to rapidly relocate to strategic points in the arena. The magic-infused stone and cooled magma that dominates the structure of the arena will force champions to work together more closely in pushing toward the enemy's nexus. It will be harder for a champion to rely on their summoner to traverse hazards and obstacles. The League is preparing this Field of Justice to be the premier arena for its use; as such, a slew of new monsters and minions await champions in what will surely be their greatest challenge yet.