how to use magic circle to trap a dragon?

[dehro]

it takes a full minute to cast magic circle... say we enter the dragon's lair... and then somehow I manage to prepare the circle before combat starts.. how do I time it so that it only triggers when the dragon happens to step into it?
is that even possible?
I mean, the spell explicitly can be used to trap something in the circle, but it doesn't really explain how that works.
Have you ever used it successfully that way, or DM'd it in use? how did that work, practically? did house-ruling come into it at all?

[GeoffWatson]

Dragons aren't affected by Magic Circle, it works on several monster types, but not dragons.

[dehro]

Dragons aren't affected by Magic Circle, it works on several monster types, but not dragons.

Well... Derp. I should've noticed.
Thanks.
That said... The question stands... What if you want to trap a creature that is affected?

[MaxWilson]

Well... Derp. I should've noticed.
Thanks.
That said... The question stands... What if you want to trap a creature that is affected?

The combo you're looking for is Glyph of Warding + Magic Circle. Glyph of Warding has two modes, one of which is Spell Glyph:

Spell Glyph: You can store a prepared spell of 3rd Level or lower in the glyph by casting it as part of creating the glyph. The spell must target a single creature or an area. The spell being stored has no immediate effect when cast in this way. When the glyph is triggered, the stored spell is cast. If the spell has a target, it Targets the creature that triggered the glyph. If the spell affects an area, the area is centered on that creature. If the spell summons Hostile creatures or creates harmful Objects or traps, they appear as close as possible to the intruder and Attack it. If the spell requires Concentration, it lasts until the end of its full Duration.

Magic Circle targets an area around a point within 10' of the caster, so it should be eligible for use as a Spell Glyph and will go off centered on the demon/whatever. Confirm first with your DM just in case to avoid any nasty surprises.

[dehro]

that seems reasonable..