The Big Lake
The Contemplator is (was) the central command figure of the Old City. It used to reside in a central pylon in the geographic center of the Old City, exerting its will via many intermediary creatures such as modrons, golems and various clockwork servitors.
The Contemplator’s purpose is to safeguard the Old City (and thus the Covenant it represents), awaiting such a time as the inherent taint of Chaos has been reduced to a nominal level, at which point it will unlock the seals which bar this world from other planes and which also bind the four Great Generals to their pylons. This will effectively reboot the war between Chaos and Law which first overwhelmed the planet.
The Contemplator’s complex in Central Command was recently overrun by a diabolic army. It is presumed that the Contemplator survives, since His destruction would also unseal the plane and the Great Generals.