I'm just going to ramble a bit about all those powerful supernatural entities D&D has running around in the Outer Planes.
Against The Wicked City talks a bit about how those demon lords & arch-devils used to be a bit closer in power level to the PCs. It is certainly true: powerful enough to be dangerous, but weak enough that a decently high level party could stand a chance of taking them out. As new editions have come and gone, we've seen these beings become increasingly more powerful.
It seems that 5e has kind of scaled it back a bit, though at CR 30, Tiamat could really ruin your party's day. Send her consorts along for the ride, & a 20th level party would probably decide to nope out, until better circumstances presented themselves.
Anyway, so far CR 30 is as high as things go, with the demon lords statted up so far being between CR 21 & 28. Of course, some of these guys used to be CR 35 or 40 or more, and I know some people don't like this downpowering.
I've decided to try to stat up a CR 30 Asmodeus. I'll be posting these stats soon. I know some people are going to be all "He's a god now. He shouldn't have stats at all or should be CR 45 at the least." A CR of 45 seems kind of pointless if you ask me.
If you can't stomach these ultra powerful beings being downgraded from previous versions, you do have options: aspects, proxies, & avatars.
Aspects are duplicates of an entity, bestowed with a small part of the being's power. They first appeared in the D&D miniatures game. They can be of various power levels and are used to represent the being in matters either too dangerous or too trivial for the entity to deal with directly. When they are created, and multiples can exist at the same time, they are of the same mind as their creator, but from that point on, they are essentially free-willed. Supposedly, an entity can create one really powerful aspect, but only one due to how much power is invested in the creation.
Aspects never willingly come near each other, and are likely to try to destroy each other if they are forced into each other's presence. If the original entity is destroyed, the aspects continue to exist, and it's possible for one to ascend to the position of the original entity.
I've never seen a proxy in an adventure, while I've seen aspects a few times. The proxy is a servant of the entity in question that has proven itself and is given a bit of the entity's power. So kind of similar to an aspect. The proxy then goes out to represent the interests of the boss.
An avatar is, I believe, a manifestation of the entity under the entity's direct control. It's not as powerful as the original, but it's still one bad mofo. If it's destroyed, the entity is all nice and safe back on it's home plane of existence.
If you've read this far, thanks. As I said, this was just a bit of rambling. Now the question is, do those stats written up for the demon lords & Tiamat represent the entities in their full glory? Or are they just avatars or aspects?