Undermountain gate question

[Halaster Blackcloak]

Well here's an interesting issue...

In the original Ruins of Undermountain box set, it says on pg. 16:

"Old, but still potent, protective magics placed by Halaster prevent many forms of teleportation and similar spells - word of recall, dimension door, succor, and even passwall - from functioning within (and into or out of) Undermountain. No magical methods of escape are possible unless such magics don't touch or pass through any stone walls, doors, floors or ceilings.

In a like manner, spells that summon creatures and items from outside the dungeon do not work. A monster summoning, for instance, only calls on creatures already in Undermountain. Most magical items that summon things from elsewhere (such as horns of Valhalla) do work.

Underlined emphasis mine. So, a horn of Valhalla would have to open a gate to Asgard to gate in valkries. Likewise, wouldn't demons be able to use their gate ability to gate in demons because they'll be coming from "elsewhere"?

[McDeath]

Well, their ability isn't a spell; so I guess. Good luck for them to get out though. Pretty sure the mage monitors such stuff in some way or form (a big TV room perhaps when he's really bored).

Sometimes I look at Undermountain like a demi-plane like Ravenloft. Its a unique place.