Looking over Warlock stuff, and I have to ask... is Investment of the Chain Master even close to worth it?
It can only be taken by Chainlocks, so it is most likely to apply to the special familiars: imp, pseudodragon, quasit, or sprite.
The Invocation has 4 effects.
It can gain a flying or swimming speed equal to 40ft -> Imps and Sprites already have 40 ft fly, Psuedodragons have 60. So, it either applies rarely when swimming matters or it applies to Quasit's that do not have a flying speed.
The familiar no longer needs to breathe -> Super rare to come up, might end up allowing them to avoid specific poisons while scouting? Poison which the Imp and Quasit are immune to. I suppose it works for holding your breath underwater, if you end up needing an extended scouting mission underwater instead of using your flight.
The Familiars weapon attacks are considered magical ->This is a straight boost by allowing your familiars to ignore the resistance of higher level enemies (or fiends)... of course, all but one are doing a measly 1d4+2/3, with a poison rider sure, but this doesn't overcome the poison resistance or immunity that most things with resistance to nonmagical damage have. (Sprites by the way get really shafted, only dealing 1 point of damage period, so this doesn't help them at all)
Your Familiar uses your DC for their Saves -> This is big, every creature on the list can inflict the poison condition on a failed save, so this is a buff. If they aren't immune of course. And, interestingly, the Psuedodragon comes out on top. They knock unconscious if the target fails by 5 or more. The Sprite gets a bit shafted, since they only knock unconscious on a result of 5 or lower, they might possibly make it up with increasing Heart Sight but... eh.
So, if you are in Imp user... this is nice for going underwater or poisoning.
Quasits can fly, go underwater and poison better
Pseudodragon go under water and poison..
Sprite... you get the idea.
So... is this really worth an invocation?
Edit: Especially since you can likely have the Imp or Sprite be invisible and just provide the help action instead of attacking for minimal damage, attempt to poison, and be visible to the enemy