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 or in the various dungeons. It literally is the only reason to enter dungeons, and even that is weak, because as said, it also is found on the surface.

Trivia

 * The name is a pun on Star Trek's dilithium crystals.