The Anarchitects were disembodied weapon intelligences used by the Enemy during an assault on Gallifrey.
These creations acted like computer viruses that were able to inhabit architecture and rewrite it, allowing them to rebuild entire cities at will. This technology was the same used by the early TARDIS models and were within the scope of the Gallifreyans, but were never considered as a weapon of war. They could infect walls and floors, but individuals were safe if they were in a river.
Lord Ruthventracolixabaxil starved to death in his own TARDIS when an anarchitect hijacked it and turned the central corridor into an endless Möbius loop. (PROSE: Alien Bodies)