How do I get the Demon's Great Ax? (Dark Souls 3)

Werewolf1986
Kill a demon in the catacombs and buy his soul from Ludlet.

Werewolf1986
Kill the demon next to the tower in the undead settlement (where there is a giant archer), or is on the wall above the wolf covenant in the swamp, or next to the Supreme Lord Volnir, and craft from the soul

Sanya Pshonkin
Kill the demon near the tower in the release of the undead
No, his soul does not fall from him.

or on the wall above the wolf covenant in the swamp.
No, it's a different soul.

or near Supreme Overlord Volnir
Da.

THX