Okay, with the Class Feature Variants coming out in less than a month, assuming that Favored Foe makes it into the book as is, I was wondering if it would be OP to give warlocks access to a features similar to that, but as an Eldritch Invocation. This would make it so warlocks wouldn't have to give up so many of their already limited number of spell slots just to cast Hex. I know there would be balance issues, especially if someone multiclassed between Ranger, Warlock, and Paladin to get Divine Favor, Hex, and Hunter's Mark all at the same time, but that would take a long time to activate, and it isn't really a ridiculous benefit to get an extra 2d6 + 1d4 on each weapon attack you make. 

Here's the proposed Eldritch Invocation:

Mark of Harming

You are a master of hexing, allowing you to more easily cast hex when you need to. You automatically learn the spell, not counting against the amount of warlock spells you know, and can cast it freely. You can cast it a number of times without expending a spell slot and without requiring concentration equal to your Charisma modifier (a minimum of once). You regain all expended uses when you finish a long rest.

I'd love to hear your thoughts, whether or not this should exist, if its balanced, and how it could possibly change. Thanks in advance. 

maybe a bit to op because of the fact that you can move the hex from target to target, and you cold have agonizing hex. other wise, love it.

It doesn't match existing invocations, which are either once per day with a slot, or infinite. 

Hex at will is very powerful when you take into account the limited number of spell slots of a warlock.  Removing concentration really puts this way over the top.

Here is a discussion of balance on a similar invocation "Accursed Memory"  that I created: https://rpg.stackexchange.com/questions/149718/is-this-homebrew-eldritch-invocation-accursed-memory-balanced

Removing Concentration from a Concentration spell like Hex is way too powerful. And it looks like you over-explained it trying to cover a lot of bases. Try this:

Mark of Harming

You can cast the Hex spell at will, without expending a spell slot or material components.

or for some added flavor

Prophecy of Zhudun

With this invocation, the warlock whispered fragments of prophecy seen in the blank face of the dead star Zhudun.

You can cast the Hex spell at will, without expending a spell slot or material components.