It's probably not an undead guardian. He's not sure whether to be relieved or disappointed. 

There are two empty alcoves the crystal could fit? Also, what school(s) of magic does it seem to have? Does the noise/breeze seem to be coming from behind the (presumably closed) door?

Crystal's got two signatures. One is basically the same as the walls and floors, but more concentrated. The other is universal and evocation, and it's pretty clearly reminiscent of an arcane mark. (Only mixed with light like an ever-burning lantern because this tradition of magic clearly does not believe in separating schools.) The alcoves have divination auras, and there's an obvious circular inset in the bottom of each which matches the size of the base of the crystal.

The breeze is definitely coming through the cracks of this door, though yes, it is closed. Actually, when he looks closer, he'll see that there are vertical stone bars sliding from the wall above and below into the door, so that to slide it open you'd need to break the stone bars reinforcing it. (At least one set from above and one from below.)

Could it really be as simple as "crystal go in hole"? Who leaves their keys just lying around? ...maybe this poor dead soul is who. One wonders what caused their demise, in that case, and how they ended up partially mummified. 

He probably could break the door down, or brute-force it with magic. But that might be construed as an act of vandalism if this place isn't as abandoned as it seems. 

He may as well first attempt to put the crystal in the nearest alcove, but he'll accomplish this task from forty feet away with a mage hand, because WIZARDS and RUNES. 

Half of the bars slide up/down and partially release both sides of the door. Now you only have to be tiny,* not gaseous, to slip through the gaps the breeze is coming through.

*Fine, technically, but who's counting.

...he levitates the crystal into the other alcove. Just in case that works. 

The bars slide back in when it leaves, and the other set do not slide out.

No thank you, he is not combing this entire fortress for another magic rock. It is time to get creative. 

He patiently casts a clairvoyance aimed about a thousand feet above his current position. 

Three thousand feet is enough to be well outside the fortress, but it's not as clear a view as you'd expect. The sensor is in the middle of a storm.

There's constant gale-force winds, irregularly gusting up to 'extreme even for hurricanes.' And there's some rain, but mostly there is dust. And rust. There are fragments of iron and bronze as powder and fragments everywhere you look, except the places they've been pulverized into plates, shields and armor battered into planks and walls of mixed metal.

Some places it looks like these battered walls of metal have been braced into covered camps that might hide people, but no people are immediately in evidence; the winds are too strong right now.

Looking straight down, there's thin enough dust and rust clouds to see the Oldwall snaking its way along the landscape. There's a section that looks damaged nearby, a piece of the outer wall hanging unnaturally and leaning on the rest.

Well, that's...deeply alarming. But he got what he wanted from the test. He finds a slightly less claustrophobic space to adopt his natural form.

Thanks to his vest of the shapechanger, a popular design among dragons who routinely interact with humanoids, some of his clothes and items meld into his natural form, leaving only an aura of transmutation and an array of passive benefits. A few remain worn on his person, however. 

Greater teleport, aiming for high enough that he won't be immediately slammed by hurricane-force winds. (He can handle hurricane-force winds, but why take the chance?) 

Now he is above the storm. It's one seriously ugly-looking storm. There's a hurricane, over five hundred miles across and only about half over land, spinning in place around some kind of fortress. A few enormous stone towers (dark stone, like the dungeon he was in) poke through the cloud cover, a ways to the southwest along the coast and maybe twice that far to the southeast. It seems to lose its force unnaturally fast along a wavy line not far north of here.

And that cloud cover is some of the ugliest you've ever seen. It's like someone ripped apart a hundred rusting scrapyards and tossed the remnants into the storm, and then it played keepaway with them rather than letting them fall to earth naturally.

Looking straight down, the clouds don't entirely block the sight of the Oldwall snaking its way along the landscape. It's big, and clearly stretches a long way east and west.

Heavens, what happened here? Not even the Font of Power unleashed devastation at such a scale. It looks like some kind of magically-induced catastrophe from the age of gods and titans. 

That fortress looks like the epicenter. Perhaps he can get some answers there. With an inward sigh, he sets out in that direction.

Ipaxalon hopes that the Pax Corps can carry on without him. They have a lot of training and contingencies, but his absence will still be a shock. And there's the worrying possibility of Netherling involvement, which could be a prelude to invasion.

If it were, though, it's a mystery why they would wait two hundred years instead of striking earlier. It doesn't seem likely they would take that long to make a move against him. Perhaps time does indeed pass relatively quickly in Earth's universe, and this influenced when the Netherlings arrived or chose to act? 

Alas, he has very little to go on. It's possible, perhaps even likely, that his unwilling travel was an unrelated fluke, and the Netherlings were not involved at all.

MEANWHILE

The ripples in the Aether caused by Ipaxalon's latest involuntary sojourn have the denizens of the Well in a frenzy of excitement. It's not often an Aetherskimmer is foolish enough to drag an entire magic system across the Aether, rather than adapting to the laws of their destination reality. Now it's happened twice in the same cluster.

There is wariness also, of course. Their agents in his previous world haven't succeeded in opening a breach in two hundred years of local time. Something is clearly amiss.

Nevertheless, this new opportunity is too delicious to pass up. They will send a variety of agents this time, with the usual arrangement: Those who succeed may partake. 

Three factions bid highest for the privilege. 

The Inhabitor prefers to scatter Its spores in search of hosts. It attempts to open a number of micro-breaches across Terratus, sending a handful of spores through each, and a single corporeal Reclaimer as well. They will seek places of power, where the mighty and the hungry may be found.

The Elder Shoals, among the most disciplined of the factions, choose the opposite path. They pool their energies and open a temporary breach for their vanguard in a relatively unpopulated region, modestly distant from where the Aetherskimmer landed. Securing territory in which to open a proper breach will be their first priority.

The Library of Fractures pens a single Sentence, a concentration of false reality so dense with power that it must be parsed into fragments before it can be insinuated, one lie at a time, into the weak places of the world. They go where they will. 

Sentinel Stand has been the home of the Regents of Stalwart for centuries. It has been besieged 44 times and repulsed 44 sieges. It is a matter of opinion whether the storm counts as the 45th, but so far it's holding out.

It hasn't stayed entirely immune to the Edict of Storms, and there are drifts of twisted metal against its outer walls and some impact craters where the metal has twisted upward, claw-like, around the edges and not been battered down to allow passage. But it looks strong, and the walls only broke in one or two places. (And in fact when he gets close enough he'll see it has faint traces of similar magic to the general abjuration on the Oldwalls. Not the same stone, this is more ordinary granite.)

The storm around it, on the other hand, is even stronger. It doesn't go up more than a few hundred feet before it dissolves into the main storm, but there is an utterly impassable wall of wind swirling just beyond the walls of the main castle. A cannon would struggle to shoot through it, let alone a catapult. Magic missile would probably work but you can really see the 'probably' on what's usually completely unerring. And that's not even considering the magic in it - the whole storm is a strong active spell effect (transmutation and universal), but the closer it gets to the center the stronger and more concentrated, and it's good and proper overwhelming here.

At the fort's gates and on its walls is an under-strength but dutiful guard in red cloth and well-made bronze armor, with heraldry bearing a symbol that looks somewhat like a snake twined around a sword. They're huddling against the wind, mostly, but when one of them does look up, they start yelling and panicking; several run for the inner keep from different directions.

Those poor people. That is an entirely reasonable and valid response to seeing an approaching dragon in the middle of...whatever this is. 

The storm makes it extremely difficult to approach the gates for a proper hail. He's genuinely unsure if he can do something about this, but it's worth a try. This calls for a freedom of movement first, though. 

He lands a good six hundred feet from the gate, braving the battering winds, and concentrates the whole of his will on control winds. The effect is strong enough to silence a tornado for six hundred forty feet around him, and by a mix of luck and magical might, he puts in enough power to handily overcome the workings of an epic archmage. 

The storm shudders.

It doesn't stop immediately. The stormwall between him and the keep wobbles, its course diverting inward and outward, and it seems like it might actually slack off.

The winds further out, for a good three hundred feet around him at least, do slack off, harsh winds blowing toward him and petering out rapidly.

But the stormwall holds. He has clearly hurt it, and it keeps wobbling somewhat. Nonetheless, it recovers, and does not fall.

(There are distant shouts of surprise and alarm from the area outside the stormwall which is now calm for the first time in a year.)

He won't try outright dispelling it without more context.

Hmm. He'd like to communicate before simply landing on the fortress, but there's a howling gale in the way.

Fortunately, he's had reason to yell at entire armies before.

His casting of mage's decree can reach up to sixteen miles, but he restrains it to a radius that covers merely the entire fortress. Then he says, to every awake creature within range, in whatever language is the most common among them:

Greetings from Ipaxalon, silver dragon of Jotenaugr, bearing no malice. I would speak with leaders. Fortress may indicate welcome by square of flags in courtyard.

And then he waits. 

It's a little hard to see what's going on in the fort, the stormwall makes it like looking through a raging waterfall, but there's motion.

There's also motion on this side of the stormwall, from two directions. The first to approach sneaks along the parapet of a badly-damaged wall that was probably part of the outer defenses of the fortress, looking out between the crenelations at the enormous beast-archon with trepidation. He's pretty stealthy, but he's used to having the constant noise and dust giving him cover, and also used to hiding from people whose senses are at most a little better than the normal human range, rather than an ancient dragon.

He's outfitted somewhat like the red-coated guards in the fortress, but his armor has seen much more recent use, his clothes are more camouflage than colored, and rather than red his insignia are dark brown.