The Curse is a hostile mob. It appears as a red, creeper-like visage surrounded by bright red enchantment particles. The curse can float, turn itself invisible, open doors, and activate redstone devices, which it will happily do in order to let in hostile mobs. The curse can also possess mobs and swords and make them hostile to the player (though this cannot effect named mobs or items).
The curse itself has 30 health and can deal 5 damage by attacking the player directly, but it prefers to avoid direct confrontation. The mob's AI is intelligent and it will try to lure players near lava or heights before trying to push them in. It may also inflict the player with blindness or nausea.
The Curse can only be encountered by using Sword Customization on an enchanted sword. While the enchantment may be transferred without issue, it's also possible for the enchantment to be removed and a curse to be spawned. The curse does not drop anything on death.