Grail levels tells you how developed a clan’s Grail is. The Grail’s level determines the Grail factor (which increases the clan multiplier). The Grail only has an effect when it is active (more than 0% charged). When the Grail is less than 100% charged, its effect on the clan multiplier is proportionate to its current charge.
The Grail’s level also affects its appearance.
The Grail’s level never decreases.