The Revenant is one of eight playable character classes in Elden Ring’s Nightreign, but it requires unlocking through a challenging boss fight. To unlock this class, you’ll need to find a unique Relic called the Besmirched Frame.
To get the Besmirched Frame, check the Jar Merchant in Roundtable Hold after completing your first Tricephalos Expedition and defeating your first Nightlord. The frame costs 1,500 Murk, which can be obtained by selling unwanted Relics during customization or rewarded from Expeditions.
Once you have the Besmirched Frame, interact with a ghostly Revenant doll in Roundtable Hold’s east wing. This will transport you back to the tutorial area, where you’ll face the Night Idol, a spectral version of the Revenant.
To defeat the Night Idol and unlock the Revenant class, focus on the Page, Pumpkin Head, and skeleton, while avoiding the Revenant’s incantations and attacks. The fight is challenging, but practicing with your chosen class will make it more manageable.
After defeating all spirits, bait the Revenant into attacking, then sidestep her rings of light to capitalize on her weakened state. Once defeated, you’ll return to Roundtable Hold, where you can choose the Revenant as one of your playable character classes.
As a complex class with low stats, but powerful magical skills, mastering the Revenant requires practice and strategy.
Source: https://www.ign.com/wikis/elden-ring-nightreign/How_to_Unlock_the_Revenant