Circle of Death is a limited situation spell, but it is already massively powerful. Why? 60 foot radius. That can be hundreds of targets. This is one of a series of war spells that can change the entire tide of a battle in one casting. Now, I'd usually prezer to have the Freezing Sphere spell with the superior range, but Circle of Death has a different resistance profile, is open to more classes and upcasts better (2d6 rather than 1d6). You're also not going to find that 'open battlefield' scenario in many campaigns, but when you do ... wow. These spells are amazing.
As a DM, I have several casters that use this spell. A powerful necromancer attacked a large town. The Necormancer's first ranks were spirits: Wraiths, Specters and Shadows. The guard rode out to face them ... and the entire guard was hit by a Circle of Death that ignored the immune spirits. The PCs then had to deal with the invading army long enough for the populace to flee.
I'm not saying that it couldn't be more powerful and still balanced - just that it doesn't need to be rebalanced as it has a powerful use that keepsit within the range of reason.
I agree the range can make it useful, but I want to note that this scenario involved the caster being an NPC likely facing enemies several times weaker than them (Circle of Death is for 11th level casters minimum and town guards are CR 1/8). This isn't a scenario that Player Character's often encounter and when they do, it often isn't something they feel like spending their most powerful spell on.
The thing is, we rarely have PCs fighting more than 20 targets. A combat with 5 PCs and 20 enemies is already fairly busy. And 100 enemies? I've never seen it.
That isn't to say that "this is an anti-army" spell isn't a good point, because I agree with you, I love the ludicrous range of this spell and the fact that I'm fairly sure it goes through cover, but that doesn't make it a good spell for player characters, who are not often nuking villages or fighting armies. So, I'd like to find a way to rebalance it that allows it to be generally more useful.
I do like the idea of the spell preventing healing. Currently the only spell that does this is a cantrip, and giving this massive area of effect a rider that if you fail the save you cannot regain hitpoints for 1 minute would make this an amazing spell I think.
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I agree that circle of death is situational but doesn't need fixing as is.
True strike: what if we made it something that was a bit like ... a clockwork amulet, or maybe even reliable talent? A roll can be replaced with a flat 10? This isn't a fully developed idea, but I thought I would toss it out
I like this idea. Maybe have it affect one attack to begin with, two attacks at 5th, three at 11th, and four at 17th so it scales with level? It still makes it kind of bad for an action though, because it doesn't change the action economy issue.
Huh.
You know Flame Blade and the other weapon cantrips have an attack as part of their action. What if True Strike allowed you to cast the spell, then make an attack as a bonus action on your turn? This circumvents the action economy issue, and it doesn't make true strike a bonus action for endless use, a mid level Eldritch Knight is still going to have to consider if this is worthwhile for them, since the turn they cast it, they don't get their full round of attacks.
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Don't forget
Find Traps. It actually beat True Strike in the worst spell ever survivor thread.
Though I do love someone figured out it can be used on legal documents.
I honestly don't know if you can fix this spell without just making it... you know the nature and location of all traps within 60 ft of you. Which isn't really something I want a caster to be able to do.