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Related: Does Mordenkainen's Private Sanctum block summoning?

I am attempting to create a scenario where a single wizard of 20th level with no special items can summon a creature of great strength (CoGS) with no risk to his own well-being.

Here's what I have so far:

At least 1 day before, cast glyph of warding at 4th level, imbuing the spell mordenkainen's private sanctum to trigger when a creature other than the wizard is located within 5' of the glyph. The side length the wizard declares is about 17-18 feet, and with the properties:

Sound can't pass through the barrier at the edge of the warded area.

The barrier of the warded area appears dark and foggy, preventing vision (including darkvision) through it.

Nothing can teleport into or out of the warded area.

Planar travel is blocked within the warded area.

The specific side length chosen is to try as best as possible not to overlap the cylinder of magic circle and cube of private sanctum, so that a creature has not enough room to be considered "outside" the sanctum, and therefore able to teleport, or "outside" the circle, and therefore able to walk freely.

Because we won't be able to see through the area ourselves, and we don't have spell save ESP, we need some way to detect that the creature is bound, so we set up another glyph of warding with the trigger "a creature allied to the wizard exists within 5' of the glyph", with the spell fireball. A CoGS is likely to survive 1 fireball, and the radius of the fireball will extend past the edge of private sanctum, so we will know.

One last glyph of warding: trigger: "a creature not allied to the wizard exists within 5' of the glyph after dusk", cast: banishment. Do that one a few times, for good measure.

On the day of summoning, the wizard takes these steps:

  1. Cast magic circle at 6th level, with the glyph of warding at the center. This will give a 4 hour duration, and therefore 4 castings of planar binding. This is done 4 hours before dusk.
  2. Move back to 55' and cast Gate, summoning the CoGS to the center of the magic circle, which triggers the glyph of warding, casting private sanctum.
  3. Begin casting planar binding at 5th level. If this succeeds, fireball triggers, and we dispel the magic circle with the CoGS under our control.
  4. Repeat casting planar binding at higher levels. Wait for fireball. No fireball? Continue.
  5. If you've bound the CoGS, cast planar binding at your highest level, instructing the CoGS to intentionally fail its Charisma saving throw.
  6. If you haven't, hopefully one of those 11 banishes will successfully send the CoGS back to its home plane.

There are a few issues I have found in this method.

  1. You cannot summon a creature with legendary resistances, as it will not reliably be bound or banished.
  2. Summoning a creature with spells like earthquake or call lightning is not risk free, because they can just target randomly and might hit you over the 4 hours.
  3. Creatures with ranged weapon attacks have the same issue, since they can spray and pray until they hit you.
  4. We're also making the assumption here that we know the true name of the summoned CoGS and that the relevant planar ruler allows the casting to pass (both requirements of gate).

What other issues are there and how can the wizard prevent as many as possible alone?

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    \$\begingroup\$ Highly related: How can I make a deal with a devil? \$\endgroup\$ – Miniman Oct 27 '18 at 0:31
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    \$\begingroup\$ What exactly is the question here? This just looks like a plan to bind a big creature but there's no actual question. \$\endgroup\$ – user47897 Dec 26 '18 at 16:04
  • \$\begingroup\$ @MarkTO What’s missing from the last sentence to make it a solid question? (I don’t see it, but fresh eyes are valuable.) \$\endgroup\$ – SevenSidedDie Dec 26 '18 at 17:11
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    \$\begingroup\$ @SevenSidedDie this seems like it might be a bit of a list question. "find and patch all of the issues in my cunning plan". There are a number of issues with it. \$\endgroup\$ – Ben Barden Apr 12 at 20:55
  • \$\begingroup\$ What's a "CoGS"? \$\endgroup\$ – Q Paul May 3 at 18:51
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You're using glyph of warding as an information exploit, probably wrongly.

If I'm reading this right, you want to use the glyph to detect whether CoGS has failed its save against planar binding. While there's no exhaustive list of allowed conditions, the spell only mentions choosing between creatures based on physical characteristics, creature type, and alignment. "Currently bound to my service" is none of those.

It doesn't say you can't have it trigger on "allied with me", but notice the example of setting a password. If the glyph could distinguish your allies from everyone else, you wouldn't need a password, would you?

(Besides which, the creature isn't your ally. You've just bound it with magic. Being unclear on that distinction is a good way to get eaten.)

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Neutralizing the confinement

A CoGS can cast dispel magic, globe of invulnerability, antimagic field, or wish or can use traits like the Rakshasa's Limited Magic Immunity or the Beholder's Antimagic Cone to just neutralize Mordenkainen's private sanctum and/or the magic circle.

A CoGS can cast magic circle at 7th, 8th, or 9th level to basically overlap your inward-facing magic circle with a more potent outward-facing magic circle.

A CoGS can transform into a creature that is not a celestial, elemental, fey, fiend, or undead with a spell like polymorph and just stroll out of the magic circle.

You might be able to prevent spellcasting within the confinement with tons of counterspell glyphs.

Dangerous even if confined

A CoGS with Blindsight can see through the fog and can target you with any of its ranged offense.

A CoGS without Blindsight can still target you with any ranged offense that does not require sight. Planar binding has a vocal component so you won't be able to hide from the CoGS. You might be able to chain silence glyphs to hide your location, but a CoGS with rechargeable AoE will force you to run away over the course of one hour just by spamming its AoE in random directions.

Preventing the planar binding

A CoGS can counterspell you if it has blindsight.

If a CoGS understand you intend to cast planar binding, it can:

  • obstruct the clear path to the target required by planar binding with a spell like wall of stone.
  • kill itself if it is more concerned with servitude than death.
  • hide within the fog so you can't perceive it and can't target it.

Odd glyph triggers

It's questionable whether you can use glyph of warding to gather information like that. A strict reading of the spell suggests that the trigger should involve some interaction with the glyph's surface/object. You should inquire with your DM.

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    \$\begingroup\$ another one shows up in the text of Planar Binding itself: "The creature obeys the letter of your instructions, but if the creature is hostile to you, it strives to twist your words to achieve its own objectives." \$\endgroup\$ – Ben Barden Apr 12 at 20:53

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