Eternium

Eternium is an interdimensional element searched by Harrison "Nash" Wells across the multiverse.

History
In 2019, Harrison "Nash" Wells traveled to Earth-1 in search of this element. He broke into McCulloch Technologies in search of it. The following day, he was met by Iris West-Allen and Cisco Ramon, who'd tracked him down. He quickly confronted Cisco, but detected it around Iris. After their confrontation, Wells located some eternium particles in a manhole.

Season 6

 * "Dead Man Running" (mentioned)