Entropic Ward is a patron of the old god feature that allows the warlock to impose disadvantage on an attack against him as a reaction. If the target misses, the warlock can give himself advantage when attacking that creature until the end of his next turn.
The PHB says that (pg. 110)
Once you use this feature, you can't use it again until you finish a short or long rest.
When is it considered used? Can I only impose disadvantage with the hopes of a miss once per short or long rest, or is it only expended once I gain advantage?
My gut tells me the former, but I wanted to make sure.