In my campaign, my players were very close to accidentally freeing Tiamat from her prison in the Nine Hells (long story). However, they ended up averting disaster and not summoning her.
However, should they have summoned her, my players had a plan which they told me about afterwards. If Tiamat were to ever appear, they planned to place a Bag of Holding next to her, and then use Mage Hand to place another one inside of it, causing this effect:
Placing a bag of holding inside an extradimensional space created by a Handy Haversack, Portable Hole, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.
Now, setting aside the difficulty of actually getting that close to Tiamat in the first place, is this something that would actually work? I'm inclined to believe not, since:
- Tiamat is enormous, and would not be fully within the 10 ft radius of the gate created.
- On her stat block from Rise of Tiamat, she has Limited Magic Immunity to spells level 6 and under. Since a Bag of Holding is an uncommon magic item, it would make sense to be immune to this effect as well.
- She's a minor deity, so she gets a pass on things like this.
Despite these points, I can't actually find anything definitively within RAW that supports my claims. Is it possible to send Tiamat to the Astral Plane using this strategy?
Best Answer
Firstly, the plan as described has one fatal flaw: Mage Hand's weigh limit is 10 pounds while a Bag of Holding weighs 15 pounds. Assuming they use still manage to use the astral tear...
RAW, there's no reason it wouldn't work
RAW typically consider a creature to be affected by effects that overlap with it despite them not engulfing the creature fully. Eg. a fireball deals full damage even if it touches just the corner tile of a gargantuan creature.
The Astral Tear is not a spell, so spell immunity doesn't help against it.
She doesn't have such protection by the rules, although it's a sensible house ruling. I'm not sure if I would enforce it myself. However...
Tiamat could probably escape quite fast
Tiamat herself lacks any ability to shift through planes, but her raw power is still enough to make her an apex predator in the Astral plane. Her extremely high Intelligence score also allows her to move faster than most creatures in there. It probably wouldn't take her long to find a creature who knows their way around the Astral Plane and the planar gateways and gently persuade said creature to allow Tiamat back from the Astral Plane. Given the potential utility of befriending a god, I wouldn't be surprised if such a creature even sought her out!
It could well be a matter of days, perhaps weeks, but assuming the party can't find a new pair of Bags of Holding by then, they'll be facing one angry Dragon Goddess once she makes her way back...