Tri-Lithium

Tri-Lithium is the fuel source necessary to power the rocket, required for interplanetary travel in Ultima II. The ship requires a single unit of this substance for every hyperwarp jump, making it a valuable commodity. Should the Stranger have ever been lost outside the orbit of earth without any tri-lithium, it may well have spelled the hero's end.

Description
Tri-Lithium could usually be found on the bodies of slain human foes, although large caches of it were stored also stored in the lowest depths and highest spires of Earth's  towers and  dungeons - a fact alluded to by an imprisoned balron in the iteration of Castle Britannia that occurred in the  late twentieth century. Finding sufficient stock of the fuel was not difficult without such ventures, however, making it likely that many of these citadels and caverns were left unexplored.

Trivia

 * A patch exists for the Apple II version of Ultima II which removes the random dropping of tri-lithium by enemies, forcing the player to traverse the dungeons and towers in order to obtain this resource. The Ultima II  of the Japanese FM-Towns port of the  Ultima Trilogy does the same.
 * The name is likely derived from the fictional dilithium crystals featured in the  Star Trek series, which are used to regulate the anti-matter-powered warp drives of star ships. Interestingly, Star Trek itself eventually referenced trilithium in the film Star Trek: Generations, wherein it was revealed as a substance powerful enough to cease all fusion within an active star. These fictional compounds, in turn, may be based on Lithium deuteride, a real world substance used as fusion fuel in nuclear weapons.