The competing invocations would be thirsting blade and improved pact weapon. I would say that favors cloak of flies, but only for a melee warlock. At 5th level this is going to be about:Ĥ * 0.65 * 2 = 5.2 average damage per roundĬloak of flies for the same character deals 4 damage per adjacent enemy. It deals Cha * hit chance * attacks per round average damage. We can also compare to agonizing blast, the gold standard ranged damage invocation. If cloak of flies affects two enemies you're looking at twice the damage, and up from there. With two attacks per round with a 65% hit chance that's an average of 4.55 damage per round, right in line with cloak of flies affecting one enemy. Hex deals 3.5 damage per hit, and uses your concentration and bonus action frequently. Let's compare it to hex, a common favorite warlock spell for augmenting damage. We also gain advantage on intimidation checks. This will usually be 4-5 damage by the time we can pick this up at 5th level. What's our reward for these downsides? Charisma modifier damage to anyone who starts their turn adjacent to us. Those are some serious downsides, in particular the out of combat one. The elephant in the room is that cloak of flies damages your allies, impedes some social checks, and can be awkward outside of combat since you can only activate it once per short rest. The only time this invocation is not so powerful is if you will be routinely gaining temporary hit points.Ĭloak of flies touches the other direction, offense. And this HP is lost first so you go through hit dice and healing magic much more slowly than anyone else. Even up through 5th level it provides 20% additional HP per fight. If you have 14 con, at level 2 this provides almost 50% additional max HP every fight. This means you effectively start every combat with 8 temporary hit points.
But you don't need to cast it during combat, and you can cast it as much as you want outside of combat until you roll a 4.
Starting with fiendish vigor, you can cast false life at will.
Two standouts to me that I never see recommended are fiendish vigor and cloak of flies. Others rarely seem to see the light of day despite being incredible options. Some invocations are recommended and utilized quite a bit, even when they're not particularly impressive.