IDEA for the reward for the final boss of the game.
Instead of a weapon, why not (if its possible) a portal that sends you to your own little pocket dimension that you can shape however you wanted. Turn it into you own magic tower, a shrine for your divinity, a cozy cottage, a shadow dimension for your captives, or a labyrinth with a boss chasing away any intruders.
All npc's that spawn in your pocket dimension are friendly to you, while you can choose whether or not they are aggressive to players you invite over.
The portal can be placed and removed easily and returns to the player once removed, no matter the distance. Think of it as an ability rather than an item.
Think of it as an extension of the game crafting mechanics after most people finish the game.