The Daedric Siege Crawler was an enormous siege machine used by the Daedra to destroy cities during the Oblivion Crisis. Its primary tool was a drill powerful enough to flatten a city's gates, though it could also use fire-balls to annihilate smaller targets such as soldiers.
The Siege Crawler was first used at the Battle of Kvatch, in which it crushed the city's defenses, allowing the Lesser Daedra to pour through and slaughter the towns-people. It was later used again during the Second Battle of Bruma in an attempt to demolish the city. Fortunately, the Hero of Kvatch managed to enter the Oblivion it was located in, took the great sigil stone, and brought it down before it could reach the city's main gate.
A notable fact is that since the Siege Crawler is powered by a Sigil Stone, when it collapses at Bruma this Sigil Stone falls out of place, and can be picked up by the player. It will have a random pair of enchantment effects, just like any other Sigil Stone.