From Halopedia, the Halo wiki
The IX-KA anomaly is the name given to a notable construction located in the Milky Way galaxy. It is known to function as a sort of prison that was employed in the past by the Forerunners. The anomaly is known to be classified as an "opson burn" and is marked by "crystalized shards of time", alongside the Kerrec Irruption and Etran Harborage.
The map found on pages 12-13 of Halo: Warfleet – An Illustrated Guide to the Spacecraft of Halo places the IX-KA anomaly in the Scutum–Centaurus Arm of the Milky Way. Given the general inaccuracies presented in this map, the canonicity of this stated location is unclear.