Eisilium is a rare element that was made-up in the 2001 Star Trek series: Star Trek: Enterprise. When Enterprise finds a comet that contains the rare element Eisilium. A Vulcan ship arrives suddenly and Capt. Vanik of the Vulcan ship tells the Enterprise crew that his crew is interested in human behavior. Eisilium was first seen in the series Star Trek: Enterprise, Season one, Episode seventeen.