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ICARUS is a proprietary scripting language/format used by Raven Software in their titles which use the Quake 3 engine. The first game to use this was Star Trek Voyager: Elite Forces. Jedi Outcast, Soldier of Fortune II, and Jedi Academy also used this scripting language. ICARUS is compiled via IBIZE.exe, which is included in the SDK. A user-friendly interface for ICARUS scripting is present in the form of BehavED.
Other Notes
Rich Whitehouse is noted for mentioning that "ICARUS would be the hardest thing to reverse engineer out of the game. GHOUL 2 would probably be a lot easier, however."