Tri-Lithium

Tri-Lithium is fuel for the rocket, required for Space Travel in Ultima II. The Stranger needs one for every hyperwarp jump. Having none while still in space usually means a game over.

Tri-Lithium is usually found on defeated human foes. It is also the content of the treasure chests found on the lowest level (level 15, so the 16th level if counting from one) in the various dungeons, and the highest level of the towers. It literally is the only reason to venture into any dungeons or towers, and even that is unnecessary, since it can also be found on the surface.

A patch exists for the Apple II version of the game that removes the ability to find tri-lithium on the surface, thus requiring exploration of the dungeons and towers.

Trivia

 * The name is a pun on Star Trek's dilithium crystals. In an ironic twist, trilithium was also mentioned in Star Trek: Generations as a substance that could destroy a star -- long after the release of Ultima II.