3.5/5 stars. Rounded down. Gain additional psychic powers thanks to not caring.
Background.
Okay. So the Emperor said some stuff at the Council of Nikaea. Magnus, the Khan, and Sanguinius said other stuff. Mortarion chimed in. And then you're no longer allowed to channel warp powers. But you don't care any more because you went rogue and don't agree with either the loyalists or the traitors and you will use the warp powers as you see fit even if they're dangerous. Think that about covers it.
Rules Evaluation.
The Warp Torrent ability is gained by both characters and independent characters. This is a nice additional shooting attack to be very candid. Some S=5 bolts that also deflagrate are cool. The range is small, but it gels well with a close combat orientated force that have transports. By taking a Perils test, you can also get AP=3 which is especially tasty, but at a price I suppose.
Finally those independent characters can pay a premium to gain access to a proper discipline. This is very handy! But you don't get lightning at all. Your HQ must be a psyker and two other characters. This is a reasonable balance that encourages you to go big. Overall then, this is a reasonable choice for your Thousand Sons wannabes, but don't choose Thousand Sons - their rules are already cool. Go for something unexpected like, erm, Imperial Fists or something?
Combinatoric Analysis.
The Eternal Vendetta: ⭐️⭐️. 2/5 stars. Too situational. But perhaps fluffy with Death Guard, White Scars, Blood Angels or Thousand Sons?
Panoply of Old: ⭐️⭐️⭐️⭐️. 4/5 stars. Useful depending on your choices.
Only in Death Does Duty End: ⭐️⭐️⭐️. 3/5 stars. A little bit average to be honest. You're already risking wounds to get AP=3, you don't need more casualties.
The Spoils of Victory: ⭐️⭐️⭐️. 3/5 stars. Pirate psychics could actually be really cool. Maybe they got trained by some eldar? Not sure what the narrative might be here, but there's tasty stuff to be had.
An Eternity of War: ⭐️⭐️. 2/5 stars. This is okay. They're mad psykers that want to be in combat. There might be better choices for your psykers though.
The Flesh Is Weak: 0/5 stars. You cannot have psykers. This combination is useless to you.
The Broken Helix: ⭐️⭐️⭐️⭐️. 4/5 stars. Psychic mutants and clones? Oh yes. This is a solid combination that I can see working really well!
In Disgrace All Are Equal: 0/5 stars. Psychic warbands not giving a hoot? Yeah, that can work! Unfortunately, you have to select a HQ, and those disgraced cannot select a HQ, so you can't have this combination sorry.
Pride is our Armour: ⭐️⭐️⭐️⭐️. 4/5 stars. Veteran psychics? Yes! This works splendidly well in my opinion. The only drawback is that the points cost is stacking up here fast with this combination.
The Taint of the Xenos: ⭐️⭐️⭐️⭐️. 4/5 stars. Psykers who have been trained by Xenos is one way of interpreting this very useful and interesting combination. I'd be tempted by this one. Especially in an Eldar direction. Some entertaining conversion opportunities are present here, along with another potential connection to the Dornian Heresy if you are tilting in that direction.
The Weapons of Desperation: ⭐️⭐️⭐️. 3/5 stars. This combination works very well as well in a narrative sense. You could imagine marines escaping from their parent legion cells or something like that?