What is technically considered an attack?

[Rebonack]

I think it would help if we knew why each feature or spell interacts with attacks. Ex: invisibility - why does this break upon attacking or casting a spell? If we knew, we could say what else might break invisibility.

An example of a trick that probably ought not work. A warlock casts Eyebite, then has a friend cast invisibility on him. The warlock can now perform three different Save or Bad Thing effects against foes, one each round. Unless the foes can find him while he's invisible, there's little they can do.

This technically works, to my knowledge. The warlock isn't casting a spell or attacking, just using an already-cast spell to do something hostile. The same could be done with many other spells.

And there are also the breath weapon and limited wand abilities above.

Invisibility doesn't say it breaks if you do something hostile; that's too vague, and would negate setting up traps which is probably an intended valid use. But as written, it enables a variety of hostile actions that it probably ought not.

And that's just one example. Coincidentally, sanctuary wouldn't block eyebite either, even though it arguably should.

You can do something similar with Spirit Guardians and a pal casting Invisibility on you. Then it's a question of how long it takes a group of mooks to figure out that there's a person in the middle of the radiant damage buzz-saw that's ripping them a new one. Basically any spell with a cast-then-concentration effect would fit into this loop-hole.

One of my big gripes with 5e is that there are quite a few things where we're told the mechanical impact without telling us what's physically happening in the game world. Just to grab Hex as an example. It deals 1d6 damage when you hit with an attack. But what's actually going on here? The best I can come up with is that it's a cursed-wound sort of deal. Every time the warlock hurts the hexed target the wound festers a bit, dealing that extra necrotic damage. But that doesn't really work, since it only triggers on attacks, not on damage. So somehow and for some reason the spell is distinguishing between the harm caused by an Eldritch Blast and a Magic Missile because one can be thwarted by nimbleness and armor while the other homes in on the target?

If we knew what the Hex spell is actually doing it might make more sense.