As we can read in Wikipedia, Thief: Deadly Shadows is a stealth game developed by Ion Storm and published by Eidos Interactive in 2004 for Xbox and PC. After Looking Glass Studios, the developer of the original two titles, went out of business in 2000, many former employees moved to Ion Storm Austin. Here they began developing the long-anticipated third part of the series, Deadly Shadows. It is the last game produced by Ion Storm before its demise in February 2005.
The idea originally was that Thief: Deadly Shadows would let you customise difficulty similarly to in System Shock, with you able to tweak how smart the AI was, what your objectives were and so on. It’s a feature which survived until the last betas, but was suddenly cut out of the final game due to the extra work it created for testers. Instead, Thief: Deadly Shadows only has the usual Easy/Normal/Expert skill settings from the older Thief games.
You can get more detail on the cut, as well as RPS co-founder and comics writer Kieron Gillen’s take on the game, in the Unlimited Hyperbole Podcast. If you have more info, screens or videos with beta differences for Thief 3, please let us know in the comments below!
Thanks to Joe Martin for the contribution!