Which season introduced the legend Revenant?

The season that introduced the Legend Revenant was Season 4.

Known as Assimilation, Season 4 began on February 4, 2020, and added Revenant as its new playable character. The season also introduced the Sentinel sniper rifle, changes to World's Edge, and a major environmental feature called the Planet Harvester.

Revenant's story was central to the season's marketing. He was originally a human assassin whose consciousness had been transferred into a simulacrum body by Hammond Robotics. After discovering what he had become, he sought revenge against the organizations responsible for his transformation.

Players sometimes confuse Revenant's original introduction with Season 18, Resurrection. Season 18 substantially redesigned his appearance and abilities, but it did not debut him as a new Legend. His first playable release remains Season 4, while his later rework was a separate update.