Gungnir or "Swaying One"

This weapon is of unknown, but likely ancient origin. Designed to level the playing field against any opponent, whether that is a mortal being or no, it possess many abilities.
The Pan Flute 5 were tasked with retrieving this artifact by Baba.  In exchange, he would grant them any godly prize he could muster.  This involved the group heading into he underdark where they would fight the Collector and take it for their own.
The weapon naturally gravitated to the Cleric of the group. This is most likely due to the weapon being drawn to him due to devotion to his deity.  Since that time the weapon has been used in many battles.
After using this, it does appear to sap the life of the user by aging them with each use of the weapon.  It is clear that the user was meant to be a diety.

Mechanics & Inner Workings

Attack and Damage

Unmissable: You have +3 Attack Bonus and Damage Rolls while wielding Gungnir. If your Wisdom modifier is higher than 3, you may use that instead of +3. Should the attack miss, the target will take 1d8 + 3 Force Damage and be staggered.
Damage: 2d8 + Wisdom Modifier, Piercing Ranged, Thrown (100/300), Versatile (3d6+Wisdom Modifier)

Passive Abilities

God-Killer: This weapon ignores all immunities and resistances to physical and a single type of damage (Radiant) 
Magic Conduit: Magic may be cast through the artifact instead of a normal focus. 
Resistance: This item gives you resistance to a single damage type (Radiant).
Power Immune: The artifact is immune to all effects and powers not cast by the wielder. This doesn't extend immunity to the wielder. It cannot be broken, transported (separately from the wielder), or altered by an opponent.
Fortify Health: You gain 30 base health while wielding this weapon.

Active Abilities

Return: This item may be returned to the user who threw or dropped the weapon last as a bonus action from any location in any plane by the most direct path. The weapon will return to the user's hand by the user's next turn if not on the same plane. The DM may rule for damage to be taken as part of its return, for example if a creature is between the spear and the wielder.
Call Bifrost: This item may, if conditions are right, allow one to summon the Bifrost to return one to Asgard or to teleport someone to any location. The user must concentrate for at least 30 seconds. A three foot wide cylinder centered on the caster will appear and teleport to a location, specific or not. Any being or object not within this cylinder completely (one part in, one part out) will suffer Necrotic damage equal to (body percentage out of the area x 10, from 1) x d6 that ignores resistances and immunities. The part will travel in the Bifrost alongside all else within this three-foot wide cylinder. You can only do this four times per day.

Item type

Weapon, Melee