The Howling Abyss can already be played on the LoL test realms, a step towards making it easier to find and play out those intense and volatile 5v5 all random, all mid skirmishes.
The Howling Abyss is designed to mimic the parameters of the old ARAM map, the Proving Grounds, with the slight change of having the entire lane flattened as opposed to having the two bases on high grounds.
The new map will be playable in custom games (where both draft mode and random mode will be unlocked) as well as through the new match-making system (which will allow only random mode). According to Riot, their plan is to make the random mode a blind one so that one team cannot see the champions of the other.
More screenshot and concept apart is provided below, courtesy of Surrender@20.
Links
League of Legends - Source
Surrender@20 - Source