Erandur sucks in a startled breath when Ruby mentions being "Dragonborn".

"Dragonborn? Well, that... sure would explain why so much seems to happen to and around you, young man."

"So, I did something at some point that impressed Akatosh and he gave me magic dragon-related powers—aren't all dragons dead? I thought you needed dragons for Dragonborn—and then for some reason I lost all of my memories, and now I have some great responsibility. I guess that beats just being a Chosen One of some sort, if the reason this stuff keeps happening to me is because of something I did."

"All dragons are indeed dead," Tolfdir confirms, "but them being alive has not been a requirement. The line of the Empire of Cyrodiil have all been Dragonborn until the death of the last Septim Emperor, and dragons have been gone for much longer than that." He frowns. "What's more confusing is why the Psijic would choose to show themselves to you now and here of all times and places. There is, as far as I know, no connection between Saarthal and the Order." Tolfdir shakes his head. "Well, let's continue exploring; maybe we will find that connection."

The reason that one coffin Tolfdir was squinting at had extra magic seems to be that it has the same effect as the wall from earlier, and the amulet Ruby found permits him to make that wall traversable for all of them.

They continue exploring, but at some point Tolfdir catches a sniff of something powerfully magical and starts chasing it.

Further up ahead, they reach large circular chamber whose walls are lined with more coffins. But more interestingly than that is that the ceiling is extremely tall, probably going nearly all the way to the surface (and maybe even further, if space was sufficiently bent), with more and more rows of coffins going all the way up.

And of course, various of those coffins explode outwards as draugar step out of them.

These are enough of them that the mages + Dremora don't immediately make short work of them. Some of them are even mages themselves, it seems.

But Tolfdir continues to be ridiculously talented, so he summons two Daedra and then creates a dome of fire around the whole room. And to justify this, a couple of draugar fall out of some of the coffins that are above them and fall directly into the fire. This doesn't immediately kill them but it really is a matter of time.

"I won't be able to hold this barrier for very long," Tolfdir observes conversationally, "so I'll only set it up periodically to weaken incoming draugar."

Reasonable enough.

Also: draugar are good target practice. Here be arrows.

"I'll remain here and deal with whatever draugar still wish to come bother us," says Tolfdir once they stop constantly showing up. "Go on ahead and explore some more, I'll catch up with you later."

"Can do," says Ruby, nodding. He walks across the room from the door they walked in through and opens the door there to keep exploring.

There are, of course, more draugar. But more than those, traps start making an appearance, too. Ruby steps on an elevated stone slab and Erandur has to quickly pull him back so that the presumably-poisoned darts that shoot from the walls at where he'd just been a second ago don't hit him. There are also magic ones, which are ironically easier for the mages to find; though visually they are very faintly glowing runes on the floor, magically they're much more obvious, and can be disabled.

Someone really did not want them to find whatever it is that's buried in these ruins, and while one part of Ruby's mind with Onmund's voice is saying that maybe they should heed these warnings and leave there's another part that's extremely curious.

Eventually Tolfdir catches up with them, with the three other Erudites in tow. "I believe I have managed to sufficiently deal with the draugar from that one chamber. I trust you have had no further problems so far?"

"Traps and draugar but nothing we couldn't deal with." Although if it weren't for healing magic they probably would be super banged up, now, Ruby's lost count of how many times he was hit.

"J'zargo is disappointed he was not here to witness your exploits. He has heard you are Dragonborn."

"Yeah, seems like it, though I have no idea what this means in practice, with all dragons dead."

(Onmund is not meeting Ruby's eyes, for some reason.)

"Let's go on, then. We're close to the magic disturbance."

From this distance it's obvious to the magic senses of all of them, and they don't even need to follow Tolfdir to find it, though honestly given the excited spring in Tolfdir's step he'd probably be running up ahead regardless.

There is a room just before the place they can sense the disturbance from which has six large metal chamfered three-sided pyramids with bas-relief carvings of one of a fish, a snake, and a bird on each side. In addition to that, the walls behind each of them have a carving of exactly one of the animals each, seemingly at random.

...it seems like it's possible to rotate the pyramids and make the outwards-facing face show the right animals? "Is this meant to be a puzzle?"

"It's probably to keep the draugar out; they are not smart enough to solve even such simple things."

And on the other side of the door that opens to them once they have "solved" the puzzle and pulled a lever leads them to a massive chamber. Their goal is very obvious, from the elevated platform they're on.

Chamber

The center of the chamber is dominated by a short platform glowing blue. Surrounding it is a magical energy field that goes all the way up into the ceiling, and floating above its center is a sphere that is the source of all the magic. In fact, it seems to be what's powering the barrier, and a more attentive look reveals that it's connected to all sorts of magical things all over the ruins. This is probably the energy source fuelling the magic barrier Tolfdir mentioned earlier. Between them and the magical sphere is a stone throne, and sitting on it is a slumped-over corpse...

...a slumped-over draugr which straightens up as soon as they walk in and fixes its glowing gaze on the group.

"Well, now, would you look at that... I never thought we'd find something like this."

Trust Tolfdir to completely ignore the draugr now standing up menacingly and the myriad other draugar that kick the doors to their coffins open and start converging on them.