Masterpiece of the Smiths (5e Equipment)
From D&D Wiki
Weapon (captain's sword), artifact (requires attunement by a creature that is not an aberration)
Once, when the Mind Flayer empire was growing across the realms, and the Far Realm threatened to consume all the worlds, a deal was struck between the Lords of the planes, to forge a weapon to turn back the madness at their gates. The blade was forged of mirhral, and coated in adamantine, both mined in the Material Plane. The crossguard and pommel, made from Astral driftmetal,a strange form of silver imbued with the innate magic of it's home plane, smuggled in secret by the midflayer's own Gith slaves out of the Astral plane. The hilt was made of Bone Devil bone, wrapped in the skin of a Balor. Once the materials were gathered, the Azers of the Plane of Fire, known for their craftsmanship, forged the blade, to be forever known as the ultimate weapon, and it was bless by the gods of the Upper planes, to create the ultimate weapon.
Astral Magic. If you cast a spell that forces a creature to do a saving throw to reduce or avert damage, and they roll a natural 1, that spell deals damage as if it was a critical hit.
Divine Blessing. Any time you take damage, roll a d8. On an 8, all allies within 30ft of you (including you) are healed by half of the amount of damage it dealt.
Elemental Synergy. Being struck with a damage-dealing spell makes your next attack deal an additional 1d6 of that spell's damage type.
Destroying the Masterpiece of the Smiths. This weapon can only be destroyed by throwing it in a Sphere of Annihilation while a God, a Primordial Titan, an Archfey, an Archdevil, a Demon Lord, and a humanoid wizard all use a wish spell to disable its magic.