Illudium is a Lambda-class T-4a shuttle that belonged to the same class of vessels as the Tydirium. This shuttle was discovered at Outpost 327 when the outpost itself was seized during the Mission to Zhar.
The names of all the shuttles located at the Outpost are derived from either actual or imaginary things. The name "Illudium" comes from a relatively unknown fictional element that is referenced in the Marvin the Martian cartoons as the "Illudium Q-36 Explosive Space Modulator".