Hmm... I would be surprised if this isn't protected by Unhallow with a Dimensional Anchor attached to it... blocking teleportation out... It also probably has Forbiddance and/or something else to block teleporting into it as well...
So I'd at least plan on that being in place... meaning we'll likely have to grab the skull and run out, then teleport away...
Dyson has Temporal Acceleration, which is basically like mini-Timestop. So I'm thinking of popping that, and running around to see if I can find it. If I do, then... what? Just grab it and run like hell? I don't think he can pick it up during the TA... so he'd have to wait until it expires...
I could use Temporal Acceleration again after grabbing it, but there's a 1 round "cool down" due to only having one immediate action (it specifically says you return to your turn that you left on)... So even in the best scenario (we run in, don't die, see the skull, and can grab it), he'd need to survive for a round and be able to get back through the hole we made...
I can't think of any better plan at this point, though...