Star Trek: Voyager, season 3, episode 16
Blood Fever
Summary
Vulcan crewmember Vorik passes on the Pon farr to B'Elanna. Meanwhile, seemingly uninhabited planet Sakari is explored. After ruins are discovered, and a few survivors are found underground by the crew, it becomes obvious that local culture was destroyed by invaders. Remains of one of the invaders are found and identified - they were Borg.
Summary from Wikipedia, CC BY-SA 4.0.
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