The Orokin Derelict is a location and tileset similar to Orokin Void. To access, single-use keys must first be crafted (Buy the BP at the Market, under Equipment). The Nav Coordinates can be found in storage containers and lockers on all missions (including the Orokin Void missions).
In the Orokin Derelict tileset, Golem Nav Coordinates can be found. They are one of the components needed to craft Orokin Derelict Assassination keys to access the Lephantis boss. Additionally, this is the only location to drop Mutagen Samples since Update 10.
You will encounter standard Infested units around level 20-30, rather than Corrupted units (which are usually enemies found in the Orokin Void). Derelicts have the greatest chance of Corpus and Grineer incursions, and with the appropriate incursion and death mark the Grustrag Three may also appear.