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Siegebreaker is purely a melee fighter, with a avalanche of a charging attack and a three-hit, punching attack 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.
Besides his gargantuan size and speed, the most talked about feature of Siegebreaker's appearance in the WWI 2008 gameplay movie were the fatalities he delivered to two of the four characters fighting him. The most memorable was the death of a barbarian, when Siegebreaker picked up the hero and outright bit his head off, then threw down the decapitated corpse with a howl of rage. In result of this action, Siegebreaker's jaw was covered with blood and body parts until the end of the battle. A Witch Doctor was similarly destroyed, not by a bite, but by a spike to the ground, after being picked up and howled at.
There's no confirmation from Blizzard, but the consensus amongst players is that these interesting moves were not special one-hit kills, but were simply coded as interesting alternative graphics. The Barbarian and Witch Doctor must have been very low on hit points, to the point that the next hit was going to kill them. So, rather than them just falling over dead, Blizzard included some special code on Siegebreaker that enables him to show off a special fatality move when one of his attacks does enough damage to cause a player death.