I'm new here, but I scrolled through and did not see this question answered. I would love to see your theory-craft at work!
What is the most damage you can do to a single opponent in one round, with spells of any class?
I play RAW, so if you are going to use any UA please have a separate category for it. No 3rd-party stuff, please.
No setup round
Damaged from anything summoned counts, as long as it attacks on your initiative
No outside help; this is about you.
Target is not restrained, or vulnerable, or resistant, unless you find a way to make him that way.
No crits unless you have a way to guarantee them.
Assume that you automatically hit, and that creatures fail saving throws.. but bonus points if you can give reasons why you have a better chance at it.
The opponent is generic, so you can't tailor the spell to a specific monster for more damage.
Use any feat, including dragonmarks to get an epic boon...
Level max is 20 using any race, class, or combination of classes
Use any magic item (as many as you like)
Spells that include a weapon attack are allowed (like booming blade)
Flame blade or spiritual weapon are also allowed
This calculation is about what you can inflict on someone intentionally, so no reaction spells. However, damage that is done on the opponent's turn in the same round can be added like that from booming blade.
I can't wait to see what you guys come up with. I'll add my own as well!