Does casting dimension door or compulsion with the magic dagger Bookmark expend your bonus action and your action?

Question

From Tomb of Annihilation, the description of the magic dagger Bookmark (p. 206) says:

While you have the dagger drawn, you can use a bonus action to activate one of the following properties:

• Cast dimension door from the dagger. Once this property is used, it can’t be used again until the next dawn.
• Cast compulsion (save DC 15) from the dagger. The range of the spell increases to 90 feet, but it targets only spiders that are beasts. Once this property is used, it can’t be used again until the next dawn.

Both dimension door and compulsion have a casting time of one action.

Does using a bonus action to activate one of these properties merely enable you to then use your action cast one of those spells, expending both your bonus action and action, or does the Bookmark essentially change the casting time of those spells to 1 bonus action?

Answer 1

Bookmark probably lets you cast them as bonus actions

I agree that this magic item is worded badly; they could have made it clearer exactly what is happening when you "activate" that "property".

If we concatenate the first sentence from your quote with the bullet points, substituting "activate one of the following properties" with the descriptions of the actual properties, we get:

While you have the dagger drawn, you can use a bonus action to ... [c]ast dimension door from the dagger. Once this property is used, it can’t be used again until the next dawn.

While you have the dagger drawn, you can use a bonus action to ... [c]ast compulsion (save DC 15) from the dagger. The range of the spell increases to 90 feet, but it targets only spiders that are beasts. Once this property is used, it can’t be used again until the next dawn.

Reading it this way, it looks like you can cast these spells as a bonus action, leaving your action free to do other things.

That said, I can see that the alternate reading would conclude the opposite. However, a magic item that lets you activate the ability to cast a spell as a separate action to the one used to activate that ability is a) very cumbersome to use (regarding the action economy) and b) not how any other magic item that lets you cast spells works. So despite the ambiguous wording, I'm going to say that reading it as I have suggested above is likely the intent.

Answer 2

Probably cast as a bonus action

I'd base how the dagger works not by the bullet points of spell you can cast, but by the other actions.

A fuller description of the dagger reads:

...While you have the dagger drawn, you can use a bonus action to activate one of the following properties:

• Cause a blue gem set into the dagger's pommel to shed bright light in a 20-foot radius and dim light for an additional 20 feet, or make the gem go dark.
• Turn the dagger into a compass that, while resting on your palm, points north.
• Cast dimension door from the dagger. Once this property is used, it can't be used again until the next dawn.
• Cast compulsion (save DC 15) from the dagger. The range of the spell increases to 90 feet but it targets only spiders that are beasts. Once this property is used, it can't be used again until the next dawn

There is no difference in wording between the first two bullet which have immediate results, and the second two which cast spells. If the spells were to act differently, such as needing an action to cast on top of the bonus action "activate", it should have said so.

Answer 3

Your bonus action only

In D&D 5e, specific beats general.

The general rule is that casting these spells requires an action. The specific rule for Bookmark that overrides the general rule, is that it requires a bonus action.