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 become increasingly red until it fills up, which is when the curse effect will occur.
The amount of curse inflicted by weapons wielded by the player is influenced by Luck, although currently, no legitimately-available item deals curse damage build-up on hit.
Instant death upon completed build-up. The animation shows the character dropping to their knees, likely from some pain before sprouting black crystals, having turned to stone.
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.