You need 5 items to be able to create the Mythic item. Location of the 5 pieces:
• Betnikh – Malacath’s Brutal Scourge Hoop – Random Monsters
• Wrothgar – Malacath’s Brutal Might Loop – Public Dungeon Boss (Old Orsinium)
• Stormhaven – Malacath’s Brutal Ritual Oil – Public Dungeon Boss (Bonesnap Ruins)
• Imperial City (Dig site is in Bal Foyen) – Malacath’s Brutal Rune Core – Imperial City Upper District Bosses
• Reaper’s March – Malacath’s Brutal Ritual Dust – Waterdancer Falls World Boss Queen of Three Mercies