"Loneliness is a slow-acting, but deadly poison" it was once stated on a tombstone in Rockland. Perhaps someone was inspired by this inscription and created the Poison Rune under this directive. Poisoning your enemy will kill it slowly but for sure, that is unless it is invulnerable to poison at all. Since this effect is passive, you don't have to worry too much about that. Either it does the trick for you or not.
This rune is nice-to-have yet not essential. Once poisoned an enemy will constantly take damage based on his maximum health and will die sooner or later so you can attend to others in the meantime.
The rune can either be embedded to your weapon in Suikoden II with a 40% probability to succeed or attached to your skin during the events of Suikoden V with a chance of 30% to inflict poison. Look for spiders to drop these crystals. - Celes