Trinity Continuum: Assassins adds a dark element to a setting that, in spite of horrors like the Aberrant War, leans toward the bright. And it does so brilliantly, with the best summation I've ever seen in any work that covers this topic: "After all, the idea that violence is universally a moral ill favors the powerful and the status quo. Those with the monopoly on violence do not want others to turn it against them. Assassins have the power to upend this status quo and strike fear into the hearts of the elite." While there are more vicious, criminal-aligned elements in the "world of Shadow" (which, yes, bears superficial similarities to certain other darkness-filled worlds Onyx Path develops), the factions more clearly intended for player membership are devoted to this ideal. Aside from inter-Assassin rivalry, the game does a fine job of creating organizations filled with despicable people. (I'm a bit surprised that the name "Theseus" doesn't appear once, given that they are already Assassin-adjacent and their arch-enemies are major players here, but there's plenty of time for their inclusion.)
By design, the Flux-shrouded society of Assassins hides beneath the pulp-inspired adventure of Trinity Continnum, and the section discussing how to connect it to other TC settings is short but to the point. Inspired Assassins gain abilities much like other Talents of the reality, inspired (pun intended) by the settings that the game is meant to emulate, but they are otherwise very much like Daredevils from previous works. High-end Assassins can perform some truly superhuman feats, but as a rule their abilties exist to let them get in, perform a flawless kill, and get out with none the wiser. Of course, missions will go sideways, and that too is built into the mechanics of the game. As the developers state up front, this is a competence fantasy, so those mechanics create opportunities for events to occur that interfere with the assassination in ways that do not arise from any poor planning or execution on the part of the protagonists. Thus, they can end up having to improvise, dodge guards they would otherwise evade with ease, or end up in dire situations that let the protagonists show off their combat skills. It's an elegant system as focused on its purpose as the spring-loaded blade at the heart of its primary predecessor.
What this is not is a game for beginners. Aside from the technical processes involved in the game mechanics, by definition TC:Assassins is about doing questionable things for a greater good and wondering if you're doing the right thing. As I have been running a one-on-one game that combines the same elements of inspiration with a setting very much like Aberrant for years now, I have tremendous appreciation for the things Assassins does and how well it does them. Mixed games that include Assassins, especially in Trinity Continuum where idealism is celebrated rather than disparaged, will raise difficult questions, and those looking for something more light-hearted should probably look elsewhere. I consider that a strength rather than a weakness, however. TC: Assassins may allow for the notion that killing might sometimes be the right thing, but it never ignores the weight of such a choice, and that balance alone makes this a great book. For those ready to embrace the Code and face tyranny, oppression, and exploitation with the most dire tool available in their gaming world, I cannot recommend this book more.