Because video games, one of the things added in the latest Diablo III patch is "The Cesspools." They're the sewers of Westmarch, a city overrun by an angel of death and his hoards of minions, making them only slightly better than a toy store in the run up to Christmas. They're included in patch 2.1.0 as a new area to explore within the endgame Nephalem Rifts, quick one-shot dungeons with totally random layout and encounters. These have also received an upgrade, now with timed "Greater Rift" versions that provide unique rewards and global leaderboards for speed and difficulty. The most significant introduction is Seasons, similar to the character ladders in ealier Diablo games, which allow a competitive version of levelling and loot hoarding.
While patch 2.1.0 (named in accordance with Blizzard's unhealthy fascination with unnecessary zeroes) was introduced earlier in the week, Seasons were held back until 8pm UK time this evening. It's in an effort to ensure exploits are found and hotfixed before they can be used to gain an unfair advantage in what is as close to PvP as Diablo has had so far. They're also staggering the launch times per region, so check this post for your time if you're not EU-based.
I've been rather heavily playing the patch since Wednesday and having a hell of a time. It's not quite as significant an upgrade as the expansion itself, but it has the game-changing impact I've not felt since MMO patch days. The new greater rifts are awesome, finally managing to make madly dashing between packs of powerful mobs fun as well as effective. Their individual loot is removed, instead dropping orbs to collect that spawn the final boss of the area faster. Once it's defeated, all that loot you would have gotten spews out of him in a humongous pile of goodies, guaranteed to contain the more powerful legendary items.
The Vault was a personal highlight, a new area that can spawn as a portal when Treasure Goblins are killed. The running loot piñatas were always one of the game's highlights, but this has redoubled the joy in finding and chasing one. It takes you to their home dimension where you can steal a frankly ridiculous amount of gold and kill their queen. It benefits strongly from the removal of the auction house earlier this year, guaranteed legendaries and riches of this magnitude not being possible when there was a market to crash.
Blizzard's run down of all the major additions is below, while the exhaustive patch notes are over here. It's reintroduced Diablo as my favourite second-screen or co-op game, so I'd recommend giving it a shot.