The Grail Knight is the title of the knight guarding the Holy Grail in Indiana Jones and the Last Crusade.
Indiana Jones encounters the knight in the canyon temple at the end of the film, after crossing the three tests that protect the path to the Grail. The knight has remained alive for centuries because he chose the true Grail and drank from it.
The character is not identified by a personal name in the film. He is presented as a medieval knight who joined the First Crusade and later became the Grail’s guardian. His long life illustrates the Grail’s power, but he explains that leaving the temple would end its effect.
A common mix-up is calling him a Templar Knight or simply the Grail Guardian. Those descriptions may sound plausible, but the film’s credited character name is Grail Knight. The character is played by Robert Eddison, whose final scene with Indiana gives the adventure its central choice between immortality and responsibility.