Trom is a fictional planet in the 30th century .
Due to the high radioactivity of the planet, the natives of Trom were never many, living in a few valleys. They all had the ability to transmute elements. As such, precious metals and other wealth had almost no value on Trom, which was a highly spiritual civilization.
The space pirate Roxxas threatened the population with extermination if they did not obey his demands (creating gold, etc). They refused, and he exterminated the entire population except Jan Arrah, who escaped, and later joined the Legion of Super-Heroes as Element Lad. The population could have easily killed Roxxas by transmuting his ships (or him) to a gas, but their spiritual beliefs prevented this.
After Roxxas was captured, Jan Arrah returned and created monuments to each of his people.
During the v4 legion, he lived on Trom alone, guarding the planet against all outside contact.
After the Legion reboot, Trom was a key part of the United Planets as the only source of the element needed to power stargates. In this version, Roxxas was a Daxamite, and with a band of Daxamites exterminated the populace.
It has not been described much in the current incarnation of the legion.