In a surprise new update for Elden Ring Nightreign, developer FromSoftware has added "Everdark Sovereign" versions of Nightlord bosses with increased difficulty and special rewards.

source https://www.windowscentral.com/gaming/new-elden-ring-nightreign-update-adds-everdark-sovereign-bosses-i-hope-youre-ready-to-get-wrecked-all-over-again