10th Level Spell Slot

In the time before the Age of Arcanum, at the height of an older and more powerful civilization known as Netheril, laid a wide collection of arcanists capable of a magic beyond comprehension to many in the modern day. Arcana of the 10th circle, otherwise known as 10th level magic, was an engine of pure control reality around a mage. Each spell of this level took decades to learn, even longer to master, and came at great personal cost to their users. This power ranged from lengething the life of their users, moving mountains, guarding and hiding cities, and even creating seas and volcanoes. If a 9th level wish is the mightiest spell a mortal can cast, then 10th level magic was the magic scratching the surface of what's expected of immortals.

Modern 10th level magic can be seen in the Lichdom ritual, Auril's Unending Winter, the 'Summon Tarrasque' spell, the release of Null from the Negative plane of energy, Elven High magic, and (though a 12th level spell) The Rite of Catatheosis. 

Even giving yourself the opportunity to cast magic at the 10th level comes at great personal or resource cost (typically a collection of thousands of souls, millions of gold pieces, death/rebirth, or several legendary magic items),  and grants a resource in a spellcasters reputoire of a singular 10th level spell slot, usable (as normally) to upcast any lower leveld spells. If an arcanist is to cast a 10th level spell, it leaves a rift in reality, summoning 1d3 random fey, fiends, celestials, or aberrations of cr 12 or lower. Additionally, 1d3 days after a 10th level spell is cast, the 60ft. radius around where the spell is cast becomes an anti-magic field, this field can be destroyed by funelling 9 levels of spell slots into the area to restore this rip in the weave.

If a caster wishes to upcast a spell to 10th level, no destructive effect or summoning occurs, but the spell is significantly altered otherwise:

• The spell gains an additional range equal to the casters level x 100ft
• The spell's Area of Effect increases by a rangeof the Caster's level x 5ft
• If the spell targets one creature (or more with upcasting), ignore rules 1 and 2 and instead effect all creatures within 100ft. 
• On spell attack rolls and saving throws made to avoid this spell, gain a bonus (caster level) to the attack roll, or a penalty (caster level) to the saving throw made by the target. The saving throw to this spell cannot be negated by using a legendary save.
• The spells damage dice are increased by one size, rather than 1d12->1d20 use 1d12->2d12
• Beyond normal upcasting damage increases, the spells damage is increased by an additional 5d
• The spell's damage instantly destroys non-magical objects and structures (dealing double damage to magical), and has explosive damage dice. 
• Upcasting this spell in this way is extremely painful and costly in the way of personal health, you take damage equal to Your HP pool - caster level - Intelligence Score) (e.g I have a total HP of 120, I'm 20th level, and have an 18 intelligence score, so I'd take 82 damage regardless of my current HP amount). In addition to this damage, you gain 2 levels of exhaustion.