D&D 5E Need help interpreting Warlock Invocations

[Hopsong]

I don't understand the spell-like invocations like Mire the Mind: "You can cast slow once using a warlock spell slot. You can't do so again until you finish a long rest." Does this mean I need to expend one of my precious spell slots? In that case, all I did was learn an extra spell that is less useful because I don't get it back after a short rest. Or is the spell not chargeable against my spell slots? I think the wording may be trying to say that the spell is cast at my current spell slot level, but it isn't clear at all. Same for Sign of Ill Omen (bestow curse), Thief of Five Fates (bane spell), and other spell-like Invocations.

As a bonus Warlock Invocation question, what is the ruling on a 5th level Fighter (gets Extra Attack)/5th level Bladelock taking Thirsting Blade to get 4 attacks per turn? Both RAW say you can attack twice "whenever you take the Attack action on your turn." It seems broken and inadvisable for a DM to allow it, but I'm curious how the wording prevents it.

 

[Satyrn]

. . . I think the wording may be trying to say that the spell is cast at my current spell slot level. . .

I like that. I like it a lot.


*yoink*

 

[flametitan]

I believe the intent is you get the spell slot back on a short rest, but not the ability to cast that spell.

As a bonus Warlock Invocation question, what is the ruling on a 5th level Fighter (gets Extra Attack)/5th level Bladelock taking Thirsting Blade to get 4 attacks per turn? Both RAW say you can attack twice "whenever you take the Attack action on your turn." It seems broken and inadvisable for a DM to allow it, but I'm curious how the wording prevents it.

No. They both say you get to attack "Twice, instead of once", not "Double the amount of attacks you have."

Likewise, the general rule for multiclassing with extra attack is "It will not work. You only get two attacks, or three if you have 11 levels of Fighter".

 

[Hopsong]

Thank you, Flame Titan. I hadn't thought about the spell slot recovery angle or that both descriptions say "twice." Both rules make sense to me now.

 

[Al2O3]

From PHB page 164 (multiclassing rules): "Similarly, the warlock's eldritch invocation Thirsting Blade doesn't give you additional attacks if you also have Extra Attack." No need to check the wording of the individual features.

And I agree with the interpretation that invocations that use spell slots are additional spells known and with more restrictions. Thus I have never been tempted to use one of those (there are plenty of other ones I prefer).