Sentinel Returns is a video game developed by Hookstone, produced by No-Name Games and published by Sony (under the Psygnosis label) in 1998, for PC and PlayStation. It is the sequel to The Sentinel by Geoff Crammond and features 651 levels, a multiplayer mode and a soundtrack (titled "Earth/Air") composed by John Carpenter and arranged by Gary McKill.
The PC version has native support limited to a software display mode and an accelerated Glide mode, because in 1998, 3DFX cards were the de facto standard for gaming 3D graphics. However, modern computers can run the game in accelerated mode with the wrappers dgVoodoo or OpenGlide, which translate Glide calls respectively into Direct3D or OpenGL calls.
This game looks very different from its predecessor. While in The Sentinel the levels were bright and colorful, in Sentinel Returns they are dark and gloomy, with flashes of light being emitted when an object is created or absorbed, and the mouse pointer dynamically lighting the world. The game has a general "hallucinated" look: the skies are made out of contrasting streaks of color; the trees look like sperms; the boulders breathe and have a sphincter on the top; the sentinels and sentries are hybrids of flesh and metal; the sentinel stands are covered with skin and have four vertebral columns protruding from the corners; the "specimen" representing the living part of the synthoid resembles a hydatidiform mole, and it squirms and lets out a shriek when injected with a needle.
Source: Wikipedia, "Sentinel Returns", available under the CC-BY-SA License.