It is shown as a plain translucent, purple chalice with a red liquid overflowing (likely Jesus' blood as told in the Bible).
On other occasion, it appears as mass of light taking the form of chalice.
The artifact has the ability to seal a god's power. It has its limits however, as it can only be used three times. When used, the national treasure requires a long incantation by a stationary person. When the incantation is finished, golden light shines down upon all in the vicinity of the grail. The light does no harm to humans, but causes 'heretical creatures' to deteriorate into nothing. In case of a powerful and unweakened heretical being, the grail can be used to 'seal' their abilities instead.
Sufficiently powerful charyeok user, such as Park Mu-Bong is capable of bypassing the incantation stage, and capable of activating it discreetly. He demonstrated it twice, once against Okhwang, and later against Jin Mo-Ri.