How powerful can spells be in Harry Potter?

Question

I've noticed in the Harry Potter series that the most powerful spells ever mentioned never go beyond killing, blowing up a few feet, or knocking someone out. Grindelwald's control of lightning in Fantastic Beasts: The Secrets of Dumbledore is the only spell I find to be slightly more destructive (because of the lightning). Even Wormtail's spell that killed 13 Muggles was aimed at something else that catapulted the number of deaths up.

I've been wondering if the strength of the spells has to do with the wand power or the wizard's skills - since the Elder Wand seems to be a factor in Grindelwald's control of the lightning.

My Question:

Is there a canon answer for the limit to how strong spells can be and [if there is a limit] what factors make it the limit?

Anything approved by JKR is considered canon by me.

Answer 1

Fiendfyre is, as far as I know, the most destructive/powerful spell described in the original books.

Fiendfyre, or the Fiendfyre Curse (Pestis Incendium), was a curse that produced enchanted flames of immense size and heat that were capable of destroying nearly anything in its path, taking the form of gigantic fiery beasts such as serpents, Chimaeras, dragons, and birds of prey that sought out living targets.

When cast, the fire appeared with a roaring, billowing noise and gave its victims only a split-second's warning to escape, quickly consuming anything in the vicinity of its caster. The flames were of an abnormally large size and take the shape of fiery monsters and beasts such as snakes, dragons, eagles, and Chimaeras, constantly mutating into other beasts as well as powerful, formless flames that destroy all things around itself. The fire even possessed a sentience of its own, a continuous (though far from latent) desire to burn anything it could. It would pursue any nearby lifeforms and anything that it could destroy, and was capable of incinerating anything through mere contact.

Considering that it is one of the few ways to destroy Horcruxes it certainly seems to be strongest spell for pure destruction. The more difficult part of casting this spell seems to be controlling the flames instead of creating them.

In the books, this spell was cast by Vincent Crabbe, who while described as proficient in the dark arts, otherwise shows no signs of being particularly skilled or possessing a special wand.