Oni

The Oni were a group of human Samurai warriors from ancient Earth who had been modified by the addition of polaric ions to their cellular structure and served the Sun Queen Himiko on the planet Yamatai. (Star Trek Online: Valkyries)

The Oni guarded several sites around Yamatai, including the monastery where Himiko was trapped and the cave where her starship was hidden (with at least one Oni being in the starship at the time of her death). The Oni would kill anyone who trespassed on these sites and in 2376 incursions by a Dominion research expedition prompted them to take the offensive and wipe the expedition out.

Nomenclature
The Oni were originally called the Stormguard, the proper name for their military organization as given by Himiko. However, at some point in the late 24th century, a Dominion expedition on Yamatai discovered them and named them the Oni after the demons of Japanese legend. The Solarii, founded by the sole survivor of that expedition, used the name as well and the Starfleet survivors picked it up from both of these groups, though they did learn the proper name of the organization eventually. ("Survivors of Yamatai")

Origin and Capabilities
The Oni were originally human warriors in prehistoric Japan. They pledged their lives to the Sun Queen Himiko, then-ruler of the Japanese archipelago. Himiko suffused their cellular structures with polaric ions, putting them in a state of temporal flux which prevented them from aging but also confused their minds and made them highly suggestible. Himiko used this as a means to control and preserve her army.

As an additional side effect to this procedure, the Oni became immune to polaron-based weapons (such as those used by the Jem'Hadar), as these were deflected by the polaric ions in their bodies. This principle was not widely understood by the Solarii who continued to try to use salvaged Jem'Hadar rifles against the Oni at until 2407, when they invaded the Oni-held monastery on the main island in an attempt to transfer Himiko into Samantha Hayashi's body.

The Oni used only traditional ancient Japanese weapons, such as swords, bows and arrows, and clubs. They appeared to have no knowledge or understanding of more advanced weaponry.

A negative side effect, however, was that the death of Himiko in 2407 caused the Oni to almost instantly disintegrate as "time caught up with them rather quickly," in Sam's words. ("Survivors of Yamatai")