You can only craft Abyssal gear in the Circle of Power. Once you have unlocked the Words of Power that teach you the Abyssal Bow and Maul recipes at your Thaumaturgy Bench, it's time to collect the required crafting ingredients to summon these weapons inside your Circle of Power. Once you start making them, your character will make a ritual, invoking a demon from the abyss that spawns over the Circle and instantly dies, so you can take the weapon off its hands and keep it.