Demon-Eye Sword is a Unique Main Hand in Diablo IV Season 15 for All classes. It comes from World drop: any Unique source - Helltide, Nightmare Dungeons, The Pit, Whisper caches, Infernal Hordes, Undercity, Purveyor of Curiosities gambling (3.2.1: "All Unique items are now available to drop at level 1, and in any Difficulty").
Where it drops
World drop: any Unique source - Helltide, Nightmare Dungeons, The Pit, Whisper caches, Infernal Hordes, Undercity, Purveyor of Curiosities gambling (3.2.1: "All Unique items are now available to drop at level 1, and in any Difficulty")