Srivani

The Srivani are a sapient humanoid species hailing from the region of the Milky Way Galaxy known to the Federation as the Delta Quadrant. They are known to perform scientific and medical experiments on other races, and possess technology that allows them to render themselves and their equipment invisible, as well as intangible.

Their experiments appear to be completely arbitrary, ranging from genetic manipulation to messing with hormonal balance of their victims. They claim, however, that they do obtain practical medical knowledge from it, and many members of their species benefit from the results.