Obtained from the giant tree located just outside of the upper exit in Firelink Shrine.
Every time a player is invaded, whether by an NPC or by another player, there is a chance that they will be able to pick a Seed from the tree. If a Seed is not available following an invasion, the probability increases incrementally until a Seed spawns, after which the chance resets.
Upon use, enemies in the player's world will react to red and purple spirits, whether they are invaders or if they are summoned via a Red Sign Soapstone. The effect lasts for 45 seconds.