Like all three generators, it was sent roughly 20 years before Ultima VII, just after the founding of the Fellowship. The effect of the Tetrahedron Generator is to twist the Ethereal Waves, so that they become sharp instead of smooth. This results in magic becoming either unpredictable or useless, while people using magic seem to become insane or senile (although their mind is still intact, only hidden). The Generator is located in the dungeon Deceit.
The Avatar was told of the Generator by Penumbra, after the Time Lord instructed the Avatar to ask her for help to enter the Sphere Generator. After obtaining and charging the Ether Ring, the Avatar entered the Generator, and, after defeating the Ethereal Beast, destroyed it. This restored magic to Britannia, giving all magic users back their sanity. This also gave the Avatar possession of the small Prism inside.
The Generator has three defense mechanisms:
- People who try to get near it, will fall asleep, and are transported farther away, before they wake up. This can be countered with the Ether Ring.
- A lightning trap directly at the entrance, shooting lightning bolts at intruders. This can be destroyed with the Destroy Trap-spell.
- If someone manages to enter the Generator despite this, a extremely powerful Ether Monster will slay the intruder.