The Revenant is one of eight playable classes in Elden Ring Nightreign. However, it's one of two classes that aren't immediately available upon starting the game.
While the Wylder, Raider, Guardian, Ironeye, Executor, and Recluse classes are all immediately playable, the Duchess and Revenant classes are locked and require you first progress a bit through the game.
In order to play as the Revenant, you first need to unlock the Duchess. Don't worry, it's not terribly hard to do so; it simply requires that you obtain The Old Pocketwatch, which you can read how to do here. Once you've unlocked the Duchess, you can begin your quest to unlock Revenant, although I must warn you that it's a bit more challenging.
After defeating first Nightlord, Tricephalos, also known Gladius, Beast of Night, in the first Expedition and unlocking Duchess, head to the Roundtable Hold and purchase the Besmirched Frame relic from the Small Jar Bazaar. Then head to the east wing of Roundtable Hold and touch the phantom of Revenant. You'll then be transported to the tutorial area where you must fight and defeat the Night Idol boss. Be careful because she summons a Page, a Pumpkin Head, and a giant skeleton. Defeating her will unlock Revenant as the eighth playable class.
Revenant is described as "the soul of a lady who manipulates spirits in her quest for revenge." She's a summoner-type class with her passive ability, Necromancy, providing a chance to revive defeated enemies as allies. Her Character Skill, Summon Spirit, will summon three different souls as backup. Her Ultimate Art, Immortal March, confers temporary immortality to living allies and revives allies in a near-death state.
Despite being one of only two classes locked to start the game, Revenant is actually considered one of the weaker Nightfarers in Elden Ring Nightreign. This is due to her dependence on Incantations or wielding specific weapons that scale with that stat, which can be difficult to come by. There's also the fact that her kit requires her to be in combat and land the killing blow and summon an ally. This is not only dangerous due to her low health, but also somewhat difficult in Nightlord boss battles where there are no enemies to revive into allies.
That said, despite her difficult playstyle, her co-op friendly abilities do make her quite useful in parties. Her support capabilities, particularly the temporary immortality and reviving, make her a valuable team member in any three-person party.