Usually found in the form of a cloud of mist, Curse will slowly build-up in a player. As Curse is inflicted repeatedly, the status bar will continue to increase until it fills up, at which point immediate death occurs.
The most well known source is the Basilisk's curse toxin. Additionally, the two Deep Accursed found in the Cathedral of the Deep and Anor Londo inflict curse buildup with every hit. The stationary Pus of Man that inhabit the Wyverns in Lothric Castle can project curse-inflicting ranged attacks onto the player.
During the battle with the Deacons of the Deep, one of the attacks in the second phase consists of the deacons generating an orb of Deep energies above Royce's staff, which travels through obstacles and causes curse buildup and intense damage. They also create a fog of gradually building curse, which can be canceled by killing the four deacons that caused it.
The third Angel of The Dreg Heap in Dark Souls III: The Ringed City will cast a beam of light at the player's location if they leave its line of sight while being attacked. The light will rapidly build up curse but does not track the player after the initial cast, and leaves the Angel motionless for several seconds.