The high priest, @, is the head of all religious activities related to a given deity. The high priests on the Astral Plane are always generated peaceful, and will remain that way unless you provoke them.
There are exactly four in the game: three corresponding to each of the deities of the player's pantheon, who appear in the High Temples on the Astral Plane, and the high priest of Moloch that appears in Moloch's Sanctum. In addition to those four, aligned priests can grow up into high priests.
The high priest of Moloch holds the Amulet of Yendor, and it is extremely likely that killing him will be necessary to obtain the Amulet. This can be difficult, and players will tend to save a wand of death charge for the specific purpose of killing him; this will not work if he is generated with a cloak of magic resistance. He turns hostile once you step into the room holding the high altar of Moloch, so be prepared before opening the door. Note that killing him before stepping in the temple (and subsequently turning him hostile) counts as murder for lawful and neutral characters.