Tri-Lithium

Tri-Lithium is fuel for space ships 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 enter dungeons or towers, and even that is weak, because as said, it also is 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.

Trivia

 * The name is a pun on Star Trek's dilithium crystals. Long after the release of Ultima II, tri-lithium was also mentioned in Star Trek: Generations as a substance that can destroy a star.