Arcane Battery

WARNING: This spell is designed to be a threat in a campaign centered around it. It could be very unbalanced, so if you consider adding this spell to your campaign, be ready to tweak it.

This spell creates an arcane battery, which can be used to fuel ones magical abilities. When you cast this spell, you need to be attuned to a magic item which isn't cursed in any way. If you are not attuned to any such item, this spell has no effect. If you are attuned to more than one item that matches the conditions, you can choose which one of the items you use. Whilst a spellcaster has one instance of this spell active and casts it again, the older instance is dispelled. Additionally, the spellcaster can use his bonus action to dispel the spell at any time.

After you succesfully cast this spell, the magic item you used as the material component gets permanently destroyed. In its place, a vibrant sphere of pure magical energy, about a feet in diameter, begins to radiate. This sphere is a magical object that is immune to all damage and conditions. It slowly feeds off of any ambient magic and grows more powerful with time. It gains different effects, depending on how long it remains active. High concentration of magic in the area speeds up the process. Also, it cannot reach certain levels, if the consumed magic item isn't of a certain quality.

Less than a 1000 days. You can use your action to touch and absorb the sphere. When you do so, you regain one of your highest level spellslots, and the sphere ceases to exist.

From 1.000 to 10.000 days (uncommon). You can use your action to refuel your magical abilities from the sphere. By expending an action and touching the sphere, you regain one of your spellslots, up to the 5th level.

From 10.000 to 100.000 days (rare). The sphere gains enough energy to increase in its size. It is now about 10 feet in diameter and can leash out up to another 5 feet. It is now permanently stuck in place and cannot be moved. When another creature starts its turn within 15 feet of the center of the sphere, they roll a dexterity saving throw. On a failed save, the creature takes 3d6 force damage, 3d6 lightning damage and is pushed 15 feet away from the sphere. On a successful save, the creature takes half as much damage and isn't pushed. You are immune to this effect.

Additionally, when you expend an action and touch the sphere, you regain one of your spellslots, up to the 9th level.

From 100.000 to 1.000.000 days (very rare). The sphere starts to increase in its size once again. Every 100.000 days, its diameter increases by 10 feet. At this stage, the sphere is extremely chaotic and is dangerous even for you. It can leash up to three halves of its diameter and it deals 3d6 force damage and 3d6 lightning damage per every 10 feet of its diameter. You have advantage on the saving throw against this effect. At this stage, the sphere is immune to the dispel magic spell - it now only temporarily stops the progression of the growth.

Once the sphere reaches 100 feet in diameter, it has accumulated so much magical energy it collapses. In its center, a dimensional rift appears, which consumes everything in a 1.000 miles radius. Everything consumed by the rift is transported to the ethereal plane, during which it takes 100d10 force damage.