The Maelstrom of Chaos was a series of random areas, with random monsters that increased in difficulty the further in you went.
You can find all the info on how and why it was created straight from GGG here.
Specifically, you are looking for the section marked:
History of Path of Exile’s End-Game
we realised that the fourth difficulty
level was only necessary until we added Act Three, at which point we’d
reduce it back to three difficulty levels so that we’re not forcing
the player to play through the same static content four times
back-to-back. Our temporary solution, a month or so before entering
Closed Beta last year, was to introduce the Maelstrom of Chaos areas.
These were a sequence of random areas (using tilesets from all over
the game) with completely random monster packs. Players gained access
to it when they finished the final difficulty and could play through a
series of levels that gradually ramped in difficulty.