I’ve talked before about why 10th level spells exist in the game world’s past but not its present. Here are some appropriately overpowered 10th-level wizard spells.
Because every such spell is lost to history (most 10th-level spell users having destroyed their own civilizations), each spell must be researched based on tantalizing clues from forbidden books. An easy Wisdom check reveals that this is a bad idea.
10th-level spells are more the purview of villainous NPCs than PCs. I bet that they can be cast with lower-level spell slots, if cast in a suitably long and interruptable ritual.
Animate all dead. Area: the world.
Detect Great Old One’s Thoughts. Your brain explodes. Everyone within 1000 miles is subject to a Feeblemind spell.
Disaster. Range: 5 miles. Everything is destroyed and everyone is killed within a 1 mile radius. Caster’s choice of meteor strike, inferno, volcano, chasm, plague, tarrasque, etc. There may be unintended lingering side effects (deadly ash, tarrasque, plague, etc)
Erase person. Range: 400 feet. Kill one person and destroy their body and gear, no save. Everyone except for the caster instantly forgets the person ever existed. Their deeds are re-ascribed to others. People with close relationships to the erased person get a saving throw after 1 week of being confused by inconsistent memories.
Immortality. The spell all the liches are searching for. Warning: may attract liches.
Mordenkainen’s dimensional domain. Warps space so that an area of up to a 20-mile radius is moved into a pocket dimension, accessible only by rules set by the caster (only through a specific wardrobe, appears one day every 100 years, etc). Don’t bring a bag of holding into this domain.
Planar blink. Duration: one hour. Every turn, you may teleport anywhere you’ve scried or visited on any plane, or you can teleport to a random location on a plane you’ve heard of but never visited, or you can teleport to a random location on a random plane. Besides the classic planes, you can visit alternate timelines, other D&D campaigns and game systems, and the theoretical dimension where game-players control your actions, so you can kill the guy controlling you.
Power word: damn. The target dies and goes to hell. They get a saving throw after a year. Even if they return, they’ll be pretty traumatized.
Soul swap. Touch someone. You permanently swap bodies, no save. You each keep your game stats but trade appearance and age.
Trans-galactic jaunt. The caster and 10 friends can survive comfortably in space and fly at 1 light-year per hour. They can finally explore the vast seas between stars and discover that, however a big deal they might be on their planet, they’re level 1 in cosmic terms.
And finally, the real reason level-10 spell research is a bad idea:
Detect 10th level spell research. The one wizard in the multiverse who knows this spell also knows Planar blink and Erase person.