What's the “magic similar to the Knock spell” referenced in the Dungeon of the Mad Mage adventure?

Question

The Dungeon of the Mad Mage module book (p. 11) has this to say about secret doors:

A one-way door can be manually opened only from one side (the other side has no handle or hinges). A knock spell or similar magic is needed to open a one-way door from the "wrong" side.

I don't get what other magic is implied to work here - what counts as 'similar magic' to a knock spell?

True seeing or a wand of secrets, for example, would let you see/find a secret door but won't help open it, as far as I know. Is there something I'm missing? Thanks in advance.

Answer 1

It's referencing any spell, item, class feature, monster ability, or effect that allows you to magically unlock a door.

In part, this is future-proofing -- you don't want some spell or magic item published in the future to fail here just because it isn't specifically a knock spell. By including "similar magic", it's left up to the DM to decide if a given effect ought to work, rather than limiting it to a single named effect. It also makes things easier on the author of the module; a little vagueness there means they won't have to go digging through every resource published to date to pick out all the acceptable options.

In terms of existing effects, a good example is the Chime of Opening, from the Dungeon Master's Guide:

...one lock or latch on the object opens unless the sound can't reach the object. If no locks or latches remain, the object itself opens.

Answer 2

The Chime of Opening is an example

For comparison, this is more specifically called out in the description of the Magic Item Daern's Instant Fortress (or Instant Fortress, if you're looking at the SRD) where it says...

It is immune to the knock spell and similar magic, such as that of a chime of opening.

source

The Chime of Opening does not actually cast the spell Knock. It does the following:

You can strike it as an action, pointing it at an object within 120 feet of you that can be opened, such as a door, lid, or lock. The chime issues a clear tone, and one lock or latch on the object opens unless the sound can't reach the object. If no locks or latches remain, the object itself opens.

Source

Why it uses generic phrasing

This is often the practice of the writers of D&D content. They are future-proofing the campaign, and proofing it against homebrew. If you simply say "This door is immune to the Knock spell and the effect of a Chime of Opening" then that opens the door for players to argue that because their specific magical toy, which was released in a book written after this given adventure was written, is not called out as not working, then it should work.

By saying "It is immune to the knock spell and similar magic" then it covers everything that exists now, may exist in the future, or may be drummed up by a creative DM.

Answer 3

A Chime of Opening may be what they're referring to here.

A Chime of Opening:

issues a clear tone, and one lock or latch on the object opens unless the sound can't reach the object. If no locks or latches remain, the object itself opens.

There may be other spells or items in the adventure itself that can open locks, but the Chime of Opening is in the DMG.