A silver Chalice that was said to have been shaped from the simple wooden cup created by Jesus Christ when he was but the son of a Carpenter, after Judas Iscariot hung and bled himself the thirty pieces of silver - drenched in his blood - were melted down and encased the wooden cup, this act of vanity gave the holy cup demonic powers and the Chalice became a relic of duality as a result: having both Demonic and Holy properties.
The Chalice was said to have been hidden away long ago by twelve of the Holiest monks (probably, the twelve disciples) in the Church's history, not even the Church and the Knights of the True Cross know its current location and many have risked their lives unsuccessfully to find it, it has become nothing but legends.
Shortly after Skoll starts staying at the Dorms and helping Rin fight Demons, there are a sharp rise in interest in the Chalice and new leads appear that hint at its potential hiding places, a hunt begins with Skoll secretly seeking to give it to his Lord Satan so that the Demon Emperor can generate and sustain a physical body in Assiah without requiring a host.
Yukio and Rin agree at the possibility that Skoll has some vested interest in the Chalice on behalf of a third party or even Satan, despite this they are curious at the blackouts and flashes of odd memories that Skoll starts having while searching through the twelve discipline temples, finally however upon touching the Chalice - Skoll goes berserk and generates Holy Fire for the first time.
Skoll gets a glimpse of his true nature back before he became a Demon and while curious of his past nature, decides that he and Satan should have nothing to do with the Chalice, which he keeps under lock & key somewhere in True Cross Academy.
The Chalice is seen for the last time where Rin is tempted into drinking from the Chalice upon seeking to lose his Demon half, which he believes to be a burden for his life as a human but drinking from it has a unique effect as he goes insane - generating out of control blue fire - and summons two black angelic wings, only Skoll upon summoning Holy Fire and two white angelic wings are able to calm Rin down and Rin finds that he has gained more control over his Demon side but in turn he has gained more terrible power than he set out to lose but is eventually reassured by Skoll that he will learn to use these new powers.
The Chalice is once again locked up in True Cross after that event, and has not been seen since. Although, there is talk between Mephisto and Arthur of using the Chalice to make more Human-Demon hybrids, and how it could tie into Skoll's future and the completion of Yuri Egin's will.
The Cup has a number of capabilities, including:
- Indestructibility - The Cup is said to be utterly and totally indestructible.
- Cursed - There are a duel layer of curses on the Cup, so much that it has to be handled with care - both when retrieving it and holding it, while within feet of its pedestal it has been cursed to destroy those that touch it. Its secondary curse is centered around those holding it, cursing them with pain and distrust wherever they go, eventually leading to their death long after they give up possession of the Cup.
- Transformation - The Cup is able to make those that drink out of it change: a evil human can become a demon, a good demon can become human, powerful Demons and good Humans can become Hybrids. The source of the Cup's power is said to be infinite, but due to its duality it is very dangerous and likely to not work as one expects. Satan, his pack and his biological children seem to be immune to the cup's power to turn demons into humans, however it is unknown if it could be used to make Satan's children into Hybrids and thereby have powerful bodies in Assiah but it is confirmed that Satan can use it to enter into Assiah without needing a human host.
- Nature awakening - It can awaken long sealed nature or traits, e.g. it awakened Skoll's dormant abilities from before he was a Demon, giving him the power of Holy Fire and later the ability to generate white angelic wings. It also made Skoll become the "Original Hybrid", the Satan-like equivalent of Demon-Human Hybrids, and allowed him to consider following Yuri Egin's will to unify Human and Demons by populating the world with his Hybrids.
- The Holy Grail is a dish, plate, stone, or cup that is part of an important theme of Arthurian literature. The Grail legend became interwoven with legends of the Holy Chalice.