I've been plagued by confusion over the nature of the demons we fight in Diablo and Diablo II. The competing factors of the soulstone, worldstone, possession of critters, and nature of the hells has lead me to a conclusion. It's not actually a revolutionary idea, but within the world of Sanctuary it is.
Demons do not have bodies.
Or rather, demons are entirely spiritual manifestations (of chaos and evil and the darker aspects of the world). As a side note, this would mean the same thing for the angels, who are supposed to be composed of pure energy.
But how can this be, I asked myself, I've slaughtered gobs upon gobs of demons in both games. Lets poke through the beastiary of Demons.
Fallen (and their variations)
Corrupt Rogues (et al)
Fetish's (also called dolls)
Megademons (traditional Balrogs)
Vile Mother (and her adorible children)
Some Baal Minions
Minions of Destruction
Theres a general trend in an increasing "outlandishness" as they become stronger and stronger (up until the Bosses and Superuniques that are different altogether).
But if we look at the initial, weaker demons, they all resemble other, non-demon creatures. Diablo 1: The fallen look like children, the goatmen look like Khazra, the hidden, megademons, and witches look like twisted humans. Overlords are said to be captured angels, Drakes look like the D2 Snakemen, and the Gargoyles could easily be actual gargoyles. The Magma and Lightning demons could be humans that had taken on elemental aspects.
In short, all of the diablo 1 demons could have been other creatures, twisted by possession. Those stronger villains can twist more (like the lightning and magma demons).
Diablo 2: Many of the creatures are shared, so I won't cover them. The corrupted rogues are fairly obvious. The tainted look like bulls or apes. Councilors, fetishes, blundergores, overseers, imps, vile mothers, and mega-demons all resemble different types of people. Those baal minions that are demons could easily be possessed versions of the animal versions of said demons. Furies are more or less humans with wings (like megademons) and putrid defilers have the upper structure of humans, horribly warped and twisted with the gas-bag-*** they have on the other end. Regurgitators give me some problem, but they too could be twisted humans. The rest of the demons out there are the special Lords, or the Minions of destruction which I assume are powerful enough to reshape their hosts however they like.
So, thats my first point: All demons resemble twisted forms of non-demons. It's no deal braker, but does lend credence to my second point. (On the side note, and I don't point to this because few of the other creatures share this type of animation. The corrupted rogue's death animations show a fleeing spirit. Perhaps it was only in temporary possession of the rogues?)
Point Two: The prime evils can't simply manifest on Sanctuary. The angels decided to imprison them in stones, force them to possess the shards, instead of banishing them right back to hell (where their spirit forms would go when their mortal coils were destroyed).
Now, I asked myself again, why would Diablo choose a child instead of a great hero (since he actually had a choice in D1)? I believe he was in a hurry. He was trying to get his brothers free and needed to be quick. His soulstone was cast down in a meteor (I think) so whoever did the casting was probably on to him. So he lured lazarus to bring him a creature that wouldn't resist his advances (the way the Tal Rasha resisted).
If Diablo had a physical body, why does he always need to possess and warp someone? Why did all the prime evils, the strongest of the demons, need bodies to possess? If they must have bodies, wouldn't a weaker, lesser demon like mere fallen or goatmen need creatures to possess as well? This could be argued to be the work of the worldstone, not the natural nature of demons, but the worldstone is such a massive variable it's difficult to be sure. I'm going to proceed as if it weren't the 'only reason'. Which it may well be. Still, for arguements sake, when you destroyed Mephisto's soulstone in hell, he didn't pop up next to you, so something must have been keeping him at bay (a lack of willing humans inhabiting hell maybe?).
So, Point Two: Prime evils need bodies, why shouldn't all the others?
Point 3: Tyrael. Now, it's possible that he has an asthetic appreciation for the human form, enjoys their pretty armors, and is really just a ball of energetic joy inside a fullplate. I submit, however, that Tyrael's form is one neccesitated by his host. Yes, Tyrael is borrowing a body, warping it with the wing-tentacles the same way the prime evils morphed theirs (to a much less asthetic degree at that). Why would Tyrael do this? Well, first is the circular logic that I'm right and he has to. But that won't stand up to scrutiny so I posit a second reason.
There are a lot more effective shapes for fighting than the two armed biped approach. If Tyrael could shape himself a form out of thin air, it'd be a lot more effective if it were ten feet tall with four arms and that could spew lightning, cold, fire, and magic alternatley. Spikes would probably help too, lot of those.
But he still has those wings, I pondered, if he can possess a body and still make wings, why wouldn't he just make all those changes like the prime evils did? I think that it's his well documented appreciation for humans. His willingness to counter the will of the high heavens in humanity's favor. Tyrael has assumed control of a person by neccesity, they may even have volunteered. He made the wings by neccesity too, what good would a mere mortal be (well, I guess we've seen exactly what good a mere mortal would be, haven't we). I think though, that he appreciates and respects the person who he's possessed too much to turn them into some horrifying freak.
Point Three: Tyrael looks pretty human, but he wouldn't have to be if he didn't need to wear a body.
So, by simple addition. If point one, point two, and point three are true, then I submit point six (1+2+3) also be true.
Point Six: Angels and Demons don't have phsyical bodies.