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:
- 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.
- 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.
- Begin casting planar binding at 5th level. If this succeeds, fireball triggers, and we dispel the magic circle with the CoGS under our control.
- Repeat casting planar binding at higher levels. Wait for fireball. No fireball? Continue.
- If you've bound the CoGS, cast planar binding at your highest level, instructing the CoGS to intentionally fail its Charisma saving throw.
- 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.
- You cannot summon a creature with legendary resistances, as it will not reliably be bound or banished.
- 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.
- Creatures with ranged weapon attacks have the same issue, since they can spray and pray until they hit you.
- 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?