You see darklight field.
Location
Boss room of Vemiath, in the Rotten Blood Quest.
Notes
It is the second stage of players tracks in the fight against Vemiath, lasting for two seconds after appearing where previously Darklight Spark was. It deals 3% of maximum health when initially stepping on it and also at each second while standing on it.
During Vemiath's fight, players with full stack of room mechanics will get hit for a total of 18% of maximum health per second.