I want to give one of my homebrew warlock patrons an eldritch invocation, but in terms of verboseness and complexity is more on par with a pact boon or subclass feature, but this particular subclass has all of its features and is supposed to follow pact of the blade. Here is the invocation (in common sense wording)

You have a list featuring the names of ten humanoids. You are familiar with all of these humanoids, wether you have met them or not. You could recognize them, even in passing and know their general area (proximity to a landmark, presence in a large town, etc.). While a creature is on the list but not the first, your attacks and spells cannot reduce it below zero hit points. Once a creature that is on the list dies, it is removed and all the names below it move up, with a new one added at the bottom.
Once per day you may speak a humanoids name, and it will appear first on the list. All other names are moved down, with the eleventh being removed.

These could also be split up into two separate invocations, with the second requiring the first