I'm building a unique weapon that can be looted off a boss creature in a very early portion of my campaign (expected for characters level 3-5), and I want to ensure that I can fairly hand it out to players without worrying about it being overpowered.
The Onyx Spear
Weapon (glaive), uncommon (unique), 8 lbs.
1d10 piercing damage; Properties: Two-Handed, Reach, Thrown (60/∞), Special
This weapon gains +2 to all of its damage rolls.
The shaft of this weapon is composed of two rods in the shape of a double-helix composed of a material that resembles wrought iron. These two rods never touch each other, but they cannot be moved closer together or pulled further apart from each other, even if this weapon's magical properties are suspended. On one end of the shaft, each of the rods are joined to blades that have the physical appearance of being carved from Obsidian, shaped like curling flames.
Special. The weight of this weapon is strange and uncomfortable, as is its momentum and inertia. Characters cannot receive their proficiency bonus on attack rolls with this weapon, even if they have proficiency with this type of weapon. Any character that makes an attack roll with this weapon may roll 1d4-1 and add this value to their attack roll.
If this weapon is permitted to free-fall, it can not fall faster than 5 feet per round. So long as a part of this weapon is touching a floor or wall, it will not tip over as a result of gravity. If this weapon is thrown, it will not stop moving in that direction until it strikes an object, whereupon it will immediately stop without imparting momentum or damage. If this weapon has not touched an object, creature, or environment in more than an hour, all magical properties of the weapon are suspended until it does.
The first time any creature touches this weapon, they need to make a Wisdom saving throw (DC 15). On a failure, they are shocked by the weapon, taking 1d4 lightning damage and dropping the weapon. On a success, they hear unintelligible whispering in their head. This saving throw is only made once per creature, regardless of success or failure.
Two-Handed. This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it.
Reach. This weapon adds 5 feet to your reach when you attack with it. This property also determines your reach for opportunity attacks with a reach weapon.
Thrown. This weapon can be thrown, using its attack roll. If this weapon strikes a target after being thrown, and the attack was not a critical hit, this weapon will deal no damage. If the attack was a critical hit, this weapon will deal damage as though it was a non-critical melee strike. For one minute after being thrown, the first creature that attempts to grab this weapon, other than the creature that threw this weapon, will take 1d6 lightning damage as they grab it.
The design considerations for this weapon are centered around the weapon feeling weird. Like there's something alien or otherworldly about its physical behavior, up to and including its physical design. It's part of a series of weapons that have similar, "strange" properties to them. The intention with this weapon (and the other weapons in the series) is not to necessarily create a weapon that's competitive with other magical items the party might acquire, just that their properties be weird and esoteric. So the weapon being underpowered is, as far as I'm concerned, perfectly acceptable.
I have a few principle concerns with how the weapon is balanced:
Unlike other 1d10 weapons, it does not have the Heavy property, meaning it can be used by Small characters without penalties. Does this have serious balance ramifications?
The +2 damage bonus is pretty substantial. This is intended to be counter-balanced by the 1d4-1 to attack rolls replacing their proficiency bonus, which on average results in a lower attack roll than simply applying proficiency. Is this enough to adequately balance the damage output?
It's not obvious to me that the other properties have exploitable features; are there concerns I should have for some of the more esoteric properties of the weapon?
I also welcome feedback on the general grammar/structure of the statblock.