The end game bosses for a campaign i'm running are a group of wizards who saved the world years ago, but they were corrupted by the Book of Vile Darkness. These wizards, one of each school, have incredible magic unique only to them, and they each have an extra spell slot they can use to cast their unique spells. For simplicity sake I just say they each have one 10th level spell slot and a custom spell for each requiring the spell slot to cast. Here is what I have so far for them. Given that this magic is supposedly stronger than spells like Meteor Storm I could use feedback on weather their under powered or over powered. 

Enchantment: Mass Power Word Kill - Basically the same effect as power word kill except it targets all creatures within a mile of the caster and disintegrates anything it kills. It comes into play as the players find entire towns where everyone is simply gone. The players will be able to protect themselves from it with some kind of magic item.

Necromancy: Call of Hades - Summons 100d100 skeletons for 24 hours, when one dies it reforms after 1d20 minuets. Any creature slain by a skeleton conjured by this spell re-animates as a zombie or skeleton and serves the caster. All conjured skeletons collapse into piles of bones when the spell ends. Requires Concentration. The skeletons are meant to be an army that can destroy entire kingdoms.

Conjuration: Summon Ultimate Being - After this spell is cast roll 1d8, then the appropriate creature is summoned for 24 hours. The summoned creature is under the casters control unless the concentration of the caster is broken, then they lose control of the creature but it remains for the duration before being sent back to its home plane. 1. Elder Tempest, 2. Zaratan, 3. Leviathan, 4. Phoenix, 5. Pit Fiend, 6. Balor, 7. Modron, 8. Solar. This spell requires concentration.

Honestly, they look overpowered. It depends somewhat on your campaign and players, of course... but even if they're all 20th level with some of the most powerful magic items in their possession, these effects still seem overpowered as something to be used in combat. For example, looking at the sort of power progression the PHB demonstrates, I'd say your "Mass Power Word Kill" is realistically closer to a hypothetical "12th-level spell," than a mere "10th-level."

Now, if they're just functioning as plot devices, with the players expected to find defenses for all these "super-spells" before actually engaging the wizards, then that's a bit different. In that case, those spells should never be cast anyway, so it's the difficulty in getting the protective items that's more important.

You might want to drive home exactly how important it is they protect themselves from these spells though, lest they get tempted to, say, skip the Enchantment protection and engage their opponents anyway.

Badass, for the "Mass Power Word Kill" I had planned on creating some magic items made to protect them from it, or some other kind of counter. As for the "Call of Hades" I planned on the players fighting through a large city filled with the undead with the kingdoms army at their back. 

I'm generally with Rod here. If you intend for the players to fight a city of undead, that's fine, but there's no need to get stressed out trying to determine the equivalent level of a spell for PCs. One reason in particular is that magical effects like these should, for most campaigns, be rare and require a lot of planning to pull off. Think Lovecraftian scenes with dozens of casters, artifacts, and the sponsorship of outside entities over years. Translating something like that into the form of a spell implies that it's something the characters can pull off at least once per long rest cycle.

A 10th level spell should only be cast once. Ever. Because your life is what powers the necessary component from 9th to 10th. Perhaps your casters use these as their ace in the hole when they have succeeded their "step" in the grand scheme of theirs.

Honestly, these are actually some really badass and awesome ideas for anyone who is going in epic levels. I know my 20th level bard would be going "This would make an awesome story!" If he saw 100 skeletons just pop out of nowhere.

I believe you mean 10,000 skeletons, being the upper limit of 100d100.

 What do you mean by modron? Do you mean any modron because modron is a type of monsters.

For anyone interested in the official D&D lore behind 10th level (and higher) spells, there's a YouTube channel, not mine, that has just done an excellent job detailing it all:

"What They Don't Tell You About 10th Level Spells" - https://www.youtube.com/watch?v=sbBkpwnFxqk

"What They Don't Tell You About 11th Level Spells" - https://www.youtube.com/watch?v=rEI0Qt3UO0Y

"What They Don't Tell You About 12th Level Spells" - https://www.youtube.com/watch?v=fp_-5kXw_9g

"How to Cast 10th Level Spells" - https://www.youtube.com/watch?v=324SELfiJxE&t=691s

While I don't plan on making use of most of this (unless my players ever want to play a high-level game, as in beyond level 20,) it's still a fascinating tale, and I still plan on basing the magical structure of the weave in my homebrew world around it.

I don't think Call of Hades needs a specific number if its a DM spell. You are just rolling an absurd amount of dice at that point, you can just say that theres waves of undead, and the party only has to get through a portion of the swarm, because the armies are holding them back. You could just roll dice for the undead the party will fight.

As for Mass Power Word Kill, if you give the party armor to keep it from killing them, it really does set the scene of all life just dying within a mile. It actually reminds me of an oddly specific thought, Final Fantasy Tactics and KO or death immunity equipment.