Suspiria was the name of the Nacene entity that accompanied the Caretaker in looking after the Ocampa. She left the Caretaker's array in the 21st century taking several hundred Ocampa with her and established her own array.
She had taken care of those Ocampa for 300 years, and had helped them to develop their psychokinetic abilities. They constructed a new array to live in while Suspiria resided in the plane called Exosia.
She encountered the USS Voyager in 2372 and ordered the Ocampa to fire upon Voyager, believing them to be responsible for her mate's death. Suspiria came aboard the ship, attacked several crewmembers and threatened to destroy the ship. Janeway attempted to explain that they had nothing to do with the death of the Caretaker, but Suspiria continued her plan of revenge. Kes attacked Tanis, the head of the Ocampa, with her psychic abilities, and Tanis' pain was transferred to Suspiria because of their mental link. Suspiria was incapacitated, and Janeway fired a toxin that subdued her. When they showed her mercy, she ceased her attack on Voyager, left for Exosia, and was not seen again. (VOY: "Cold Fire")
Suspiria was initially conceived of as essentially a form of "get out clause", designed to change the format of Star Trek: Voyager if such a need arose. This was because the studio executives at Paramount were wary of audience reaction to the lost-in-space concept that was central to the series. During Voyager's first season, executive producer Rick Berman explained, "The studio was very concerned when they first heard the pitch. They felt the idea of the ship being so far away from home was a bleak premise... a hopeless premise. It wasn't quite "out there" like Star Trek is used to. It's "getting back." We convinced them that it didn't have to be bleak [....] And frankly we made a concession to finally finish the sales job... we put the one-armed man out there–which is the other entity that we met in the pilot. It's out there somewhere. We will try to find that entity more than once during the next several years because we know that the entity has the ability to send us back home." (A Vision of the Future - Star Trek: Voyager, p. 192)