The Revenant was one of Alex Wesker's B.O.W. developed on Sejm Island using the Uroboros Virus. During the "cultivation" experiment with the Uroboros, a scientist noted that if the Uroboros virus were to be injected into a t-Phobos infected corpse, that corpse would be reanimated with a core as its weak point - this means that Revenants are technically t-Phobos/Uroboros hybrids.
In a note found in the hidden facility, it is stated that the security needed to be both powerful and continuous in order to prevent any experiments from escaping, as the subjects could potentially wipe out the island's inhabitants. This may suggest that the Revenants had been contained by any security forces (automated or human) until around Barry Burton's arrival (six months later), where the Revenants finally managed to escape. A few are still seen in containment, possibly suggesting that they either escaped, but only the ones that weren't contained, or that they were released into the island and a few were kept for emergencies, like an intruder or large numbers of Revenants dying and needing replacement.
Revenants were sealed away inside the hidden laboratory beneath the mine, escaping (or possibly being released by Alex Wesker) approximately six months after Alex Wesker's suicide. This made life on the island much harder for Moira Burton and Evgeny Rebic, as Revenants would seek out and slaughter all living creatures, were far harder to kill than the Afflicted, and did not share the aversion to strong light that caused the Afflicted to hide away for most of the day.