Can a player hold the effect or spell granted to them by an item, and if so, does the spell or effect follow the normal "Readying spell" rule where if the trigger never occurs, the effect or the spell, or the charges on the item are wasted.
IE: "I hold the effect of my Cape of the Mountebank (dimension door) until the minotaur moves into melee with me."
The minotaur never moves into melee with said character.
While wearing it, you can use it to cast the dimension door spell as an action.
Spells Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell and caster level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn. When you ready a spell, you cast it as normal but hold its energy, which you release with your reactions when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.
Using the above quotes (with some unrelated text omitted, and with my emphasis in bold), I see this situation with the following logic:
Therefore, if the trigger does not occur, the spell is lost. You must wait until the next dawn to use it again.
Note that this question explicitly asks what happens when the effect of a magic item is held via the Ready action. For the more general case of readying the use of a magic item, see this question.
You ask two subtly different things. Readying the use of an item's effect to cast a spell is much different than casting a spell.
You use "ready" as your action to use the cape's effect to cast Dimension Door if a trigger occurs. The trigger doesn't occur before the start of your next turn, the cape's effect is never used and you may still use it.
You use "ready" to ready a spell to hurl at a target if a trigger occurs. You cast the spell and hold back the spell's energy as per the PHB (pg 193). At this point, the spell is cast and the spell slot consumed. The trigger does not occur before the start of your next turn, so the spell dissipates with no effect.
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I believe the best way to state that would be:
"I ready an action to 'Use an object' to activate my Cape of the Montebank once the minotaur gets in range"
If the trigger never occurs than interacting with the item (which is what expends the charge) doesn't happen and the charge isn't spent.
Use an Object: You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn.
