The Holy Grail, also known as the Chalice, is a cup that Jesus Christ used in the last supper. The Grail is powerful and celestial, and houses unlimited divine powers.
During the Last Man Standing arc, the grail was being fought over by King Arthur's men and Myrddin. This continued on till the late 20th Century, where Myrddin forces John Constantine to find it, or he will kill the last living decendant of King Arthur, Rich and his family.