Poison is a status effect introduced in Riviera: The Promised Land, and behaves typically in that it saps the victim's health. Poisoning is a status effect affiliated with the Darkness element, therefor many Dark-based attacks/skills can cause poisoning.
In Riviera - The Promised Land, Poison saps 16% of the character's max HP each time they get to move. It lasts for many counts, and can be reset to the beginning if the character takes another attack that poisons them again. Getting afflicted by Poison when already poisoned does NOT stack the damage. It's an annoying status effect, but manageable with proper healing and/or a cure such as an Elixir. No charactes are immune to Poison. Ein and Fia have no resistance to Poison.
In Yggdra Union - We'll Never Fight Alone, Poison is a rather nasty status effect. It inflicts gradual damage during a clash, which is stacked with the damage being taken normally from the opponent. Also, it reduces the Morale of each afflicted unit by a small amount at the beginning of their turn. As a result, a poisoned unit will have a hard time winning battles; when losing battles on top of the morale damage taken at the beginning of the turn, the accumulative morale loss can be quite severe. It does not wear off until the current objective (Condition of Victory) is met; use Refreshment to cure the unit of this ailment.