Bosses
Mistress of Pain
It has been revealed that this boss might have a huge role in the storyline.
The Mistress of Pain is an arachnid demon and wears a kind of old, torn Victorian Dress and a Head-dress. Her face was covered with a veil in her first concept art but another picture reveals just how hideous she really is, as well as detailing some more striking features.
Her legs are her most salient feature. She has three pairs of proper legs along with a pair of stingers in the front. The legs do not look like any kind of insect’s legs as they are made of soft flesh rather than an exoskeleton, appearing more like the thighs of a voluptuous woman. Her shins are covered with black stockings decorated with intricate golden demonic designs.
Her abdomen is also very soft but is covered by her rib cage, which extends down to cover her entire rear-end for protection. On the tip of her abdomen are spinnerets that spin a very sticky web that can doom unwary victims.
Another, very unusual, feature are the openings seen on top of her abdomen. She can lob dangerously toxic projectiles towards her enemies through them. What other purpose they serve is currently open for debate.
The Mistress of Pain can be half-confirmed to be present in Diablo III as either an Act Boss or a super unique due to the various references to the player in her second, detailed concept art.
She has been revealed to have some soft parts which might indicate that the player must be in a certain position to do damage to her. Also notable is the rib cage covering her abdomen, which appears to be breakable to reveal her soft insides that could be easily damaged.
The concept art shows she has a normal slashing attack from her frontal stingers. She can also secrete sticky threads to slow or even stop the player in his tracks. She also has a unique ranged bombardment attack that is released from one of her openings on her back, which drops clusters of damaging projectiles on the player.
Siegebreaker Assault Beast

The Siegebreaker Assault Beast is the first large boss monster revealed in Diablo III and it blows away all of Diablo II’s Act Bosses and Uber Monsters added together, by sheer mass and power.
He is so large that the concept of a single character fighting him is kind of laughable. It is like a mouse taking on a large dog, except that this dog has six legs with spikes on the front two. Of course the mouse has magical spells, armor, and weapons, so maybe it is not such a mismatch after all.
Siegebreaker is purely a melee fighter, with an avalanche of charging and three-hit, punching attacks that deals horrifying damage with the huge blades on the back of his hands. He has some sort of stomp or trample attack as well, one that tears up the earth and creates clouds of smoke from the earth in front of him.
Thousand Pounder

Thousand Pounder is a special boss monster, seen in the WWI 2008 gameplay movie and a few related screenshots of Diablo III. He is seen in the Old Cathedral beneath Old Tristram.
The Thousand Pounder may be one of the many random event special mini-bosses, that the player may encounter in his adventures. This game mechanic may have been reused by Blizzard seeing the popularity of the Act V guest monsters in Diablo II: Lord of Destruction. The player may eventually be able to fight against many more Thousand Pounders as the game progresses through higher difficulty levels. The Thousand Pounder seen in the gameplay video could be some kind of super unique member of his kind.
But it still cannot be infered whether he is one particular individual out of a whole class of big, fat, dual-wielding, tattooed demons called Thousand Pounders, or if he is a special one-of-a-kind demon like the Diablo II Super Uniques, especially after his small caption seen under his name, Gluttony Incarnate, which suggests the latter. In a piece of Diablo III artwork, more than one Thousand Pounder can be seen, but as artwork does not always reflect in-game content, this does not confirm or infirm this monster’s unique status.
He is not an act boss or a major quest boss though, that much was made clear in the panels at the WWI event, much before the new revelations.