A level 11 warlock wakes up. They capture or spot a small creature. They hex it, and then they kill it.

Then they take a short rest.

They are now concentrating on Hex and have all of their spell slots back. This hex concentration lasts 24 hours.

Their at-will damage goes from 31.5 to 42, a 33% boost. This lasts until they need their concentration for something else, or lose concentration.

You can also impose disadvantage on either strength/dex (if the target is likely to be grappled) or spellcasting attribute (if they have counterspell) checks.

If you spot a foe before combat starts, you can impose dex disadvantage to give them disadvantage on their initiative check.