Alien artifact

Zeratul: "I bring tidings of doom. The xel'naga return, the cycle nears its end, the artifacts are the key."

Jim Raynor: "The key... the key to what?"

Zeratul: "To the end of all things."

- Zeratul delivers a warning to Jim Raynor^src

Xel'naga crystal

Alien artifacts are xel'naga-crafted artifacts somehow associated with the return of the xel'naga.

At least some of these artifacts could be considered dangerous, since they can manipulate the minds of terrans. At least some are alive in a sense (once being described as an "energy fossil").

The protoss (especially the Dark Templar) were interested in collecting these artifacts, but the zerg and the Terran Dominion was also interested in them.

By 2504, the Dominion had banned trade in "alien goods" in order to control access to them.

The complete xel'naga device

During the Second Great War, the Moebius Foundation hired Raynor's Raiders to retrieve artifacts across the sector in competition with the Zerg Swarm. The artifacts were actually components for a xel'naga device. The device was used to deinfest Kerrigan at the Battle of Char and then installed in the Korhal Palace as a security device against Infested Kerrigan for Emperor Arcturus Mengsk.