Ok so to understand this you have to read the Ascendant Raisin grimoire card. These raisins are obviously evil. From the text, it is safe to assume that the raisins drain the Light from Guardians, which is why it has to wait to become Ascendant. The longer these raisins are in our possession, the more powerful they become. By the time of the next raid, the raisins will have enough power to form a dangerous creature capable of destroying the Last City.