The Tartarus Debris Field is a massive debris field in the galactic core, beyond the Omega 4 Relay. It surrounds the Collector base and is composed of destroyed starships that attempted to navigate the relay without a Reaper IFF. Joker comments that many of the ships look ancient.
The debris field is essentially the Collector base's first line of defence. Few ships can navigate the field safely without suffering massive hull damage, as normal kinetic barriers can't handle the number of impacts involved. If a ship does manage to enter the field, the debris also hides several dormant Oculi, which will pursue the invading ship until it is destroyed.