Teleportation circle (PHB, pg. 282) can be made permanent, as the spell description says:
You can create a permanent teleportation circle by casting this spell in the same location every day for one year.
Dispel magic (PHB, pg. 234) can only target an object, creature, or magical effect.
A permanent teleportation circle doesn't seem to be an object or magical effect; it is an inscription, a writing. Can it actually be dispelled?
As Jeremy Crawford indicates here.
If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise.
Later this makes it into the official Sage Advice Compendium
Can permanent magical effects be dispelled? Or are they no longer considered magical effects once permanent?
If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise (such as in the wall of stone spell).
So unless a spell description says otherwise it is still an effect of the spell and dispel magic ends spells.
The logic behind RAW as I see it is that dispel magic ends spells. A spell effect that becomes permanent is still an effect created by a spell, thus can be ended with dispel magic unless more specific verbiage (e.g. the text of a specific spell) states otherwise.
A Teleportation Circle would be a magical effect (it is the effect of a spell, after all). There is nothing in the spell description that says making it permanent changes this.
And Dispel Magic specifically says it can target a magical effect as you point out.
So this would work.
On the other hand, it seems a bit harsh for a year of work to be cancelled out by a single casting of Dispel Magic! As a DM I might declare that a Teleportation Circle made permanent is now a magic item rather than a magic effect and therefore not affected by Dispel Magic (or possibly only temporarily suppressible). But this would be a homebrew ruling.
Let's start with what Dispel Magic says:
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a successful check, the spell ends.
Keeping that in mind, let's see what a magical effect is. According to Sage Advice:
Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
If your answer to any of those questions is yes, the feature is magical.
Note that there are two types of magic in D&D. See the question from Sage Advice (Is the breath weapon of a dragon magical?) for an explanation.
So through the list, a permanent teleportation circle is not a magic item, is not a spell attack, and its description does not say it's magical. There are two options that seem plausible. First, "Is it a spell?". I'd argue that it is not a spell, since you cannot simply cast a spell and create one (with the exception of with Wish, but that's a separate issue). It can only be created as a result of casting 365 spells over the course of a year. The other is, "Is it fueled by the use of spell slots?". While its only use involves using spell slots, the portal that is opened is what's fueled. The permanent circle is simply a "marker" for the teleportation circle. Since it is none of these options, it is not a magical effect per se and cannot be dispelled with dispel magic.
A second argument, if the first didn't convince you: What does Teleportation Circle do? Let's look:
As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle [...] A shimmering portal opens within the circle you drew and remains open until the end of your next turn.
So the effect of Teleportation Circle is to create a portal to another point in space. While RAW doesn't explicitly state this, it is strongly implied by the following that a permanent circle is not a portal:
Many major temples, guilds, and other important places have permanent teleportation circles inscribed somewhere within their confines. Each such circle includes a unique sigil sequence - a string of magical runes arranged in a particular pattern.
I'd love an official source for this point though.
Back to the argument, since the effect of Teleportation Circle is a portal, and a permacircle is not a portal, I argue that the permacircle is not "the effect of a spell becoming permanent" which would warrant the answer that permanent means until dispelled.
One last point on the logical side: One may claim that the sage advice ruling was an oversight, and should include any effect that was created as a result of spells. To them I ask: Can you destroy the berries from Goodberry with Dispel Magic?
The following is my personal interpretation of the spell.
I believe that there's no magic holding a "Permanent Teleportation Circle" in place. I believe it to be more simply like a marker, perhaps a "dent" in the Weave so to speak. It doesn't teleport anything; rather, the normal Teleportation Circle does. A permanent circle simply serves as a marker for the spell to "lock on" to. The magic lasts 6 seconds each time the spell is used, but beyond that what is left is not a magical effect.
A teleportation circle is the effect of a spell.
Effects of spells are magical effects.
So you can dispel a teleportation circle; the DC is 15. Arguably the wizard could cast the last teleportation circle at a higher level to boost the DC slightly.
However, it is worse than that.
Many major temples, guilds, and other important places have permanent teleportation circles inscribed somewhere within their confines. Each such circle includes a unique sigil sequence--a string of magical runes arranged in a particular pattern.
A permanent teleportation circle must have a unique sigil sequence -- a string of magical runes arranged in a particular pattern -- inscribed somewhere within their confines.
Thus, if you remove the unique sigil sequence, there is no teleportation circle there anymore. Whatever material the circle is inscribed on can be vandalized and the circle destroyed.
(Permanent Teleportation Circle) implies (Sigils Inscribed), thus not (Sigils Inscribed) implies not (Permanent Teleportation Circle). So either destroying the Sigils destroys the Circle, or the Sigils cannot be destroyed (which seems to be stretching things).
So Dispel Magic is just slightly faster than a mundane with a pickaxe.
The question the destroyer has to ask is, do they want to destroy a wizard-year of work and almost 20k of components invested? Or do they want to learn another destination.
