In my games there's magic that's quick and easy, and magic that requires organisation - groups of casters, sacrifices, rituals, etc. Anyone who meets the requirements can cast them.

To name some examples of organised magic:

My players got four pieces of hag magic. They each require three arcane casters to cooperate, each spending a relatively low level spell slots to get a high level effect.

They also found a war rite from a clan of giants, which requires five or more warriors to consume a hallucinogenic mushroom concoction and sacrifice some blood. They may expend up to three hit dice, rolling them and taking that much damage. The next time they enter combat they get the rage effect with a bonus equal to the number of hit dice spent. This has no restrictions and no duration, but the rage keeps them from backing down until all enemies are dead - while they are aware of enemies, they must take either the attack or dash action.

Finally, they found a ritual to create a zombie plague. After making some rather morbid sacrifices and some 5th level spell slots, a group of necromancers can make all zombies in a region infective. The radius in miles depends on the number of participants - every uneven participant adds one mile, so with five you get a three mile radius. All participants must be capable of casting animate dead and contagion.

It's honestly really fun to come up with these, and while my players have only cast the hag rituals a handful of times, and used the giant war rite just once, I think they appreciate that they're allowed to cast the "npc only" spells.