The level requirements for Warlock invocations stated in the PHB are refering to which level, character or Warlock class?
Example: If I multiclassed into Warlock, Monk6/Warlock2, would I be able to pick up One With Shadows as an invocation which carries a 5th level requirement?
The PHB errata clarify this as well.
Eldritch Invocations (p. 110). A level prerequisite in an invocation refers to warlock level, not character level.
Except where noted in the multiclassing rules, when you gain class features depends on your class level. In this case, invocations are class features that don't have special rules in the multiclassing rules that say otherwise, so you get them according to your class level.
Note that in the case of invocations, it's even stronger than "it's not mentioned in the multiclassing exceptions": the rules for invocations specifically call out your warlock level (PHB, p. 107, "Eldritch Invocations", emphasis mine):
When you gain certain warlock levels...
when you gain a level in this class...
If that isn't clear enough, the specifics of how you gain and choose invocations directly references your warlock level (all emphasis mine):
When you gain certain warlock levels, you gain additional invocations of your choice...
This next quote is especially telling, since it explicitly refers to the rules' notion that the current warlock level is what determines which invocations you can learn, and would be nonsensical if character level is what mattered:
Additionally, when you gain a level in this class, you can choose one of the invocations you know and replace it with another invocation that you could learn at that level.
Everything about invocations references warlock level, not character level.
The pre-req is your warlock level, not overall character level.
Here's the thing about this, if you interpreted these conditions as character level, basically all classes' descriptions would break down. Here's a few examples:
Evocation wizard (phb 117):
Beginning at 2nd level....
Wild Magic Sorcerer (PHB 103)
Starting at 6th level...
Assassin Thief (PHB 97)
At 13th level...
Basically, classes never refer to class levels directly. They always imply that you're dealing with class levels. You don't gain class features for levels you didn't take. The warlock invocations are the same way, the level listed is a class level pre-requisite.
While it may be debatable as to whether this is legal per RAW, it's been pointed out by the designers of the games that they at the very least intended for it to require warlock levels, not character levels.
Of course, you can choose to rule otherwise, but note that this makes a 2-level Warlock dip very attractive to any high-level character when you can pick up stuff like Master of Myriad Forms, Visions of Distant Realms, and especially Lifedrinker right away.
For example, a Fighter 11/Warlock 3 can easily be getting an additional 3-4 damage with every attack, which may not sound like much but is actually quite a lot in this edition. Keep in mind that a typical Sword 'n Board fighter deals an average of 8.5 damage with each attack even at level 20 with 20 strength, and even a Great Weapon Fighter only deals ~13.3.
Well most people like looking at this as a player level or class level problem. As a DM I look at the balance. The Eldirtch Invocations say 5th or 12th level specificity isn't noted where most things are noted by the Wizards of coast and are clarified. For example for those who would say that they simply forgot about the implications of the Eldritch invocations, they specifically reference the thirsting blade aspect with extra attack. From game balance though I think that level requirements is so that lower level characters can't have them, than the people who didn't dedicate enough levels to warlock. My example is similar to how spells are treated in D&d, and that its player level and not class level
