The Sentinel is a computer game originally developed for the BBC Micro in 1986 by Geoff Crammond. It was later ported to other 8-bit computers like the Commodore 64 (Crammond himself handled the conversion), Amstrad CPC and ZX Spectrum as well as 16-bit computers like the Atari ST, Commodore Amiga and IBM PC.
It's best known as one of the first games to use polygonal graphics, which -- due to the unique nature of the game's controls, in that the player is always stationary -- didn't require an absurd amount of computing power (at the time) to render. It still ran a lot more slowly on 8-bit systems than on 16-bit systems, however, though fans were known to boost the computing power of older systems to be able to play it more effectively.
The surreal premise of the game is explained minimally: The player controls a consciousness that inhabits an immobile vessel called a Synthoid, which can look around the multi-tiered 3D chequerboard world, but can't move. What it can do is absorb and create objects (which gains and spends energy respectively) that is on a square that it can see the entirety of (that is, it can't absorb or create anything on a square above its line of sight) or on one of the platforms it has created, known as "Boulders". The player can also create more Synthoids, and can instantaneously transfer their consciousness to them, at which time it will be facing its now unused Synthoid (which it will most likely absorb along with whatever else the fresh Synthoid can see from the new vantage).
The player's goal is to transfer itself into the eponymous Sentinel at the highest point on each map where it can then Hyperspace to a new level. The Sentinel is systematically turning in place and scanning its field of view for anything with more than 1 point of energy (everything except Trees) -- it absorbs any such object once found and redistributes the energy as Trees around the level. This includes the player's Synthoid, which means avoiding the Sentinel's gaze is a top priority.