"Go to Hell," says Hagan; his snake hisses in a way that almost sounds like laughter.

:You could tie a rock or something to the end of a rope and drop it through and then pull it back and see if it's still intact? Maybe have Van do it with Fetching while you're all in there: 

He can pull a rope out of his backpack and arrange this.

Vanyel suggests they have the Unseen Servant close the door, then they can slip out to tie one end of the rope to something in the central room, and then shut their door again and open the new door and he can use Farsight to guide his Fetching and drop the rock through. 

The rock drops into the room with the stars and - meanders off through it, in the direction it was shoved in. Soon it's out of sight.

Vanyel waits a bit and then uses Fetching to tug the rope back and see if the rock is still on the end of it. 

Yep.

"Hmm. Could we send a summoned creature through with a rope harness? If something happens to them there, they'd still just go back to where they were summoned from, right?" 

"Yeah, they would. I need you to refresh that spell, though."

Vanyel can do so. "Remind me what the time limit is on it?" 

"The longest-lasting one I have is just over two minutes. That's probably long enough to see if they die of anything, though."

Then as soon as the creature is summoned, Vanyel can nudge the stars-door shut, slip out and re-tie the rope, dart back, and then nudge them through, count to sixty, and pull the rope back. 

Does the summoned creature seem all right? 

The summoned creature seems to disappear when nudged through the door but is still there when tugged out. 

"- that's the result you'd get with an antimagic field. I think he made a demiplane with no gravity and no magic - I don't know why you'd do that, though -"

"Because it's cool? You've got to admit that it's extremely cool."

"I mean, it'd prevent anyone from getting at his possessions using magic, right?" 

"Yes, but it'd also be awfully inconvenient for him to get at them." Sigh. "Not sure what to try next. We could go back to Sothis and get a chicken or something?"

"That is weird. Maybe he had a completely non-magical arrangement for getting at it? And, sure, we could do a non-summoned living creature next." He'll feel a bit bad if they kill a chicken but not that bad, since chickens purchased at a market in Sothis presumably get eaten most of the time. 

"We should think about other things we want before we burn either two Teleports or two Gates - uh, if the chicken lives, is that enough to convince us to try sending Fy in -"

"The place gives her the creeps and she's not sure she can maneuver in it."

"I could...turn her into a bird?"

"You can d– right, of course you can do that. If Fy doesn't want to go in, though, I don't think we should send her against her will - I could–"

:No, you are NOT volunteering to go through the creepy door into the place where magic literally doesn't work!: Pause. :This isn't a serious suggestion, but we could wait for some other foolhardy party of adventurers to try it and see what happens to them. I guess if it turns out to be easy then they get the loot first. Still, it seems like it'd take away a lot of the advantage that Van gives us, if magic doesn't work there: 

"We could absolutely just leave it," says Mahdi regretfully. "I just don't want to, I bet it wasn't cleared out of easy-to-move stuff after the pharaohs died like the other ones were."

" - I mean, if you think there's more than five grand in there in expectation then it's worth it for me to go in even if it's pretty likely to kill me."