The Grand Arcanum is a 2003 anime exclusive method to make a Philosopher's Stone originating from the people of Ishval that calls for at least 10,000 human souls. It was used by Scar to make a Philosopher's Stone out of Reole late in the series. It's also briefly metioned in PS2 game Curse of the Crimson Elixir as the art used to create Golems.
It would appear that the Ishvalan Exile was either the creator of or only known person knowledgeable of the Grand Arcanum. He first taught Scar's brother how to make it when he was researching alchemy, and later taught Scar himself.
The Grand Arcanum is used by drawing a very large Transmutation Array around an area (similar to the Nationwide Transmutation Circle from the manga). When activated, the Grand Arcanum will take the souls of everyone inside the array and turn them into a Philosopher's Stone. It also requires that the user tattoo their body with special texts and designs, effectively making them a living Philosopher's Stone by drawing all the souls into their own body.