The spell Mind Sliver's text says:
You drive a disorienting spike of psychic energy into the mind of one creature you can see within range. The target must succeed on an Intelligence saving throw or take 1d6 psychic damage and subtract 1d4 from the next saving throw it makes before the end of your next turn.
The Pseudragon's Sting attack reads:
Sting. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 4 (1d4 + 2) piercing damage, and the target must succeed on a DC 11 Constitution saving throw or become poisoned for 1 hour. If the saving throw fails by 5 or more, the target falls unconscious for the same duration, or until it takes damage or another creature uses an action to shake it awake.
The warlock Pact of the Chain, Investment of Chain Master feature says:
If the familiar forces a creature to make a saving throw, it uses
your spell save DC.
So, if I am a POC warlock with 20 CHA and a pseudodragon familiar, I can Mind Sliver an enemy before having my pseudodragon sting them. This seems to essentially increase the already high CON DC by 1d4, but it's actually more powerful than that because the modifier gets applied after advantage/disadvantage. Mind Sliver also increases the DC to remain conscious by that amount, respectively (so long as they aren't immune to poison).
Imagining that I had the foresight to buff the pseudodragon with invisibility before sending it into battle, it would make the attack with advantage (against enemies that can't see invisible creatures). There are many other buffs that a PC could add as well, e.g. Armor of Agathys.
Is this interpretation of interaction between traits and features correct, and if so, is it too powerful?
Of note: game balance is not limited to interactions "in the wild" or "on the campaign trail". This has implications for many social interactions as well. Knocking someone unconscious for an hour is a great way to infiltrate their barracks / steal their magical inventory / kidnap them / etc.
Best Answer
Probably not
While this is a cool combo, I do not think it is "game breakingly overpowered".
First, poison is the damage type with the highest number of resistant or immune creatures. I count over 200 from the various Monster Manuals and extension splat books that are immune to poison.
Second, many powerful monsters have legendary saves. You invest invisiblity, your action to cast the cantrip, and possibly other buffs to make this save happen. If they are not immune to begin with, even if they fail they can just decide to succeed.
Third, don't forget the action economy. Whatever you opt to spend your action on is what you do for the round. With fights on average taking only 3-4 rounds, in any serious fight you are trading off casting the cantrip instead of casting your high level, high impact spells. It might be a good option once you've run out of those, which can happen with a warlock.
You have to separate the Timmy effect ("look how awesom it is if I can stack all these buffs and features after I spend multiple rounds preparing it") from the actual contribution of the core combo.
In effect, you already have a hard DC just from being a warlock and having a pseudodragon. That is the powerful part here.
It is nice to make that DC even harder, and it may increase your chances by an average of 12.5% to have the sting connect. But getting these 12.5% costs you your action. I do not think it is broken, or even the best thing to do with your action in many cases.