Wearing this ring in the Grave of Saints or the Doors of Pharros will summon other players into the host's world. The player can be hollow or human for the summoning to work. Once other players are in the host's world when summoned this way, the Crest of the Rat cannot be taken off until they depart.
It can also grant a flat bonus to the Poison buildup stat on poison weapons when worn. The bonus granted from these items can vary depending on what type of poison weapon is equipped, be it innate, infused or both: