It is not required viewing, but may help.
The first thing needed is a deep understanding of the game's Stealth mechanics.
"Stealth mechanics" is a vague term, but what I actually mean when I say this is, "You need to understand as much as possible about everything that interacts with Stealth or affects Stealth in this game."
Going a step further, what *that* actually means is, understand everything that affects *Detection*, since Stealth is actually your Default State. Normally you're in Stealth all the time anyway, so you don't really do anything to "stay" in Stealth. A better way to think of it is that you make mistakes or allow things to happen, which *change* your state into Detected.
Understanding how Detection works, what causes it, what slows it, and what stops it is the first step.
In AC Unity, guards use a three-phase Detection/Awareness Meter. For our purposes, it's a two-phase meter since the third phase is being Exposed/causing an Alert, which for this kind of video is considered an instant failure.
In Phase 1: The guard sees the Assassin, and their Awareness begins filling with Yellow. Breaking Line of Sight with the guard before this phase fills up will cause them to stare in the direction you were last seen in, but they will not Move from their position. Phase 1 filling with Yellow completely will transition to Phase 2.
In Phase 2: The guard has looked at the Assassin for long enough that they're starting to become super suspicious of them. In Phase 2, the guard's Detection Meter begins filling from the bottom again, but this time, with Red. Breaking Line of Sight with the guard before this phase fills up will cause a Last Known Position Hologram to appear, and the guard will then walk to that spot to investigate who you are. Phase 2 filling with Red completely transitions to being Detected/causing an Alert. Playing with this Phase is important, as it allows you to manipulate guards. It also feels like playing with fire, as staying in sight for too long voids your run.
NOTE: In Assassin's Creed Unity, there are special zones called Restricted Areas. Gameplay in these zones is the same as everywhere else, with one exception: Guards who spot the player IMMEDIATELY go into Phase 2 Detection. The premise is that, since they've been told to keep everyone out regardless, even a glimpse is enough to get them instantly suspicious.
In this video, I use this knowledge, combined with knowledge that the Guard will move to my Last Known Position, to let a Brute glimpse me for a moment so I can drag him out toward a doorway and assassinate him there, right before dual assassinating his two allies inside the building.
Killing or Knocking Out a guard who is about to Detect you stops detection.
Killing a guard in plain sight of another guard causes instant detection.
Disappearing out of sight of a guard who is about to Detect you halts detection, but their meter stays filled to whatever point it filled to before vanishing, until they decide your presence was probably nothing, and just forget about you.
Throwing a Smoke Bomb to block Line of Sight between you and a guard prevents the detection process from beginning, for that guard who can't see you.
Throwing a Smoke Bomb onto a guard or group of guards prevents the detection process from beginning for all guards who are coughing/affected/stunned, for a few seconds. You can use this time to move through the area, or kill all hostiles before the effect wears off.
Throwing a Cherry Bomb causes guards who are BOTH in range and have line-of-sight of the impact-point (where it landed) to turn to look in that direction, and if it's far enough away, to walk over to Investigate it. This can be used to manipulate and redirect their attention elsewhere, and either neutralize them while they're not looking, or move through the area before the effect wears off.
Being inside a thick crowd prevents Detection as long as the player is walking slowly and does not press the Run/Sprint button. Doing a Low Profile kill while inside a thick crowd makes the action invisible to nearby enemies. Entering High Profile (holding R2/RT/Right Mouse) nullifies this crowd Blending effect.
These are the primary things a player needs to know about Stealth in Unity and Syndicate.
In AC Unity players have access to four (4) basic Assassination Animation Types.
They also have access to Air Assassination, which is considered High Profile by the game no matter what.
Low Profile Moving animations tend to be smoother and focused more on a quick kill, after which the player can keep moving smoothly. High Profile Moving animations tend to be more brutal and flashy-looking, and in Unity and Syndicate it's a crapshoot whether you'll get one that's good for continuing to flow forward or whether you'll get one that stops you in your tracks. I use many Low Profile Assassination animations because they're shorter, end faster, and are generally better for letting you keep moving around quickly. Some exceptions are: The two guards in the beginning, I High Profile Kill both of them, and the sleepy guard standing beside the "care" building, I High Profile Kill him too. One of the rooftop guards is High Profiled as well, because High Profile Kill animations have a greater range, and can quickly "lunge" at enemies who may be able to turn around and spot you.
While on Ledges, you can either do a Low Profile Kill, which will pull the target off the ledge, but has short range, or a High Profile Kill, which leaps onto the surface above while killing the target in one motion. Understanding when you can safely do a High Profile Ledge kill also helps facilitate smooth movement, since you've effectively climbed onto a higher part of the map while killing a guard at the same time - it's optimizing movement and kill-flow at once.
A comprehensive post on understanding and correctly using different Kill Animations in Unity and Syndicate is something I'm considering making, since I already have a ton of it written out.
Some targets are either unreachable, or it would take too long and ruin the flow and pacing of the video to go out of your way to eliminate them through hand-to-hand or melee stealth kills. In this video, there are two guards in such positions that I shoot using Unity's silent Ranged weapon: the Phantom Blade. They're both Gunners standing guard on balconies. Gunners have the lowest Health out of any other enemy archetype in the game, and no matter what Level they are, they are always instant-killed with a single bodyshot from a Phantom Blade. Memorizing this allows a player to feel confident in quickshotting them, as opposed to some enemies who need to be shot in the head, since a bodyshot won't kill them, and will therefore make them Detect you.
After learning as much as you can about a particular game's Detection mechanics and the rules that they intersect and interact with, now comes time to practice.
Most of these Stealth videos are not accomplished in a single attempt. At the very least, some understanding of the mission and of the guard layouts is needed, so the missions themselves are often studied by the player until they have a good grip on timings and the locations of all the guards. Players who enjoy Stealth games often make videos like these as a sort of "Custom Game-Mode" or "Custom Mission" when they're finished doing everything else in the game but want to find a reason to keep playing a game they still enjoy. The systems and mechanics are all there, it's just that the self-imposed restrictions of never being seen and of trying to get a fluid/cool-looking run take those mechanics and use them in a player-created challenge to create more replay value.
On TheHiddenBlade.com, such player-created challenges are the bulk of what we specialize in and celebrate.
The higher your understanding of the game's mechanics and systems, the less times you will need to replay or "drill" the mission to get it right. These skills are also Transferrable between Stealth games. Which is to say, the more of this kind of video you do, the better you get at it overall, across all Stealth games. I've also done Stealth videos of Wolfenstein: The Old Blood, Dishonored, The Last of Us, and the AC Chronicles sub-series of stealth platformers.
Playing Non-Lethally involves the same kind of comprehension of a game's Detection mechanics, but it's arguably much more difficult to do. Many players may scoff at this or not fully understand why not killing anyone could possibly be harder, since, when going out of your way to assassinate every enemy in an area, there are more chances for the player to be seen. However, Killing a guard in a Stealth game, at its simplest, means that you are removing a threat. When playing Non-Lethally, either the player is choosing to leave all of their threats standing without eliminating them, or they're choosing to take them down in less harmful ways, which in most Stealth games are a lot slower-animated than hitting a guard with a Kill Animation.
Essentially, when a player plays Non-Lethally, they are sending the game, themselves, and their audience, a message: "I'm so skilled, I'm willingly leaving all the ways in which I can be Detected active, or willingly taking extra risks to my Stealth state to take those threats down without any kills."
This playstyle can sometimes involve playing very aggressively (or, "flirting") with Detection meters. This is because, due to leaving threats active, it's common to be in *someone's* line of sight most of the time, always on the brink of alerting everyone.
You choose which fantasy sounds more badass and appealing to you. The Ghost? Or the Reaper? Perhaps both? I recommend that at some point in your gaming career, you try playing both styles, since your understanding of one will contrast against your understanding of the other, making them both more fun and meaningful in the end.
If a game is sophisticated enough to offer a Non-Lethal Takedown option (in AC Unity and Syndicate this is the Choke, done by holding the Drop/Interact button in place of a standard Stealth Kill), then having knowledge of Timings and guard patterns is important to make sure you have the extended time you need in order to get away with doing one. Non-Lethal play will demand every bit of your skill, and you'll need to have a solid grasp on how different tools and abilities you control affect guards. The tension tends to sky-rocket while playing Non-Lethally since there are very few ways to "save" yourself from an Alert if you're in the wrong place at the wrong time.
The Choke can neutralize a guard without killing him, allowing you to stay Non-Lethal, despite the animation taking longer.
Smoke Bombs can remove the ability of guards to Detect you, just long enough for you to sprint past them before they come to their senses, or long enough to choke-down their buddy and quickly leave.
Cherry Bombs or Knives can make guards look somewhere else, while you slip past behind them. Indeed, the token AI Manipulation or Distraction item in Stealth games is the main item the player usually needs to learn in order to play Non-Lethally.
Bricks/Bottles in The Last of Us, Noisemaker Arrows in Thief, Magazines in Metal Gear, Cherry Bombs in AC Unity, Knives in AC Syndicate.
Sadly, Assassin's Creed Unity and Syndicate are not built or designed with Non-Lethal gameplay in mind, unlike more "serious" Stealth games like Dishonored or Metal Gear Solid. Because of this, playing Non-Lethally is harder in these games than perhaps it should be, and is really only something you'd do if you either loved them and are looking for extra challenge, or you feel like showing off. With luck, future entries in the Assassin's Creed series will validate Non-Lethal play as a viable style, and give players more tools to pursue it with. The Main Targets must always die, but the same doesn't need to be said for their grunts and guards.