Killer stealth builds? (Esspecially for Legion if possible)

There are a few buckets of perks that will be useful to you:

• Any perk that gives Undetectable, other than Beast of Prey, will help you approach survivors stealthily, so using Tinkerer, Dark Devotion, Insidious, or (to a lesser extent) Trail of Torment will give you something to work with. Nemesis and Hex: Retribution are this category as well, because they applies Oblivious, which is very similar to Undetectable.
• Perks that give you specific information about where survivors are will also be useful, since you need to know where survivors are in order to know where to jump scare them. For example, perks like Hangman's Trick, BBQ & Chili, I'm All Ears, Nurse's Calling, Surveillance, Discordance, or Spies from the Shadows will all give you precise information about where survivors are without them knowing you know. I'd avoid obvious information-gathering perks like Infectious Fright, because you want the survivors to be unaware that you know where they are.
• Terror radius reduction can allow you to get closer to survivors before they know you're close. This is extra strong on killers with a naturally small terror radius. Monitor & Abuse is great for this purpose. Furtive Chase is less useful but it can let you make some unexpected plays, especially if you're chasing near another survivor that's doing an objective.

Now for killer-specific thoughts:

* Ghostface doesn't really need a build around stealth, since it's already in his base kit. Just use his power.

* For Nurse, I'd recommend running range add-ons with her. She moves slowly when not blinking and has a large terror radius, so stealth isn't really her strong suit. You can still definitely jump scare survivors by blinking through walls from far away, though!

* For Spirit, you could try running the Prayer Beads add on. This will make it so that survivors hear the whooshing noise when you start phasing everywhere on that map (except when they're within your TR). Rather than knowing from the whooshing noise that Spirit is coming, then, they'll only be able to trust their eyes.

* For Oni, I'd recommend using his speed add-ons and/or his ultra rare add-on that shows survivor auras when they come into contact with a blood orb. He's one of the least stealthy killers in the game when he's using his power, but you can still jump scare survivors because of his high movement speed. I'd recommend approaching gens from off angles while using your power and zooming into view at the last second. This only gets easier with his ultra rare add-on, since an injured survivor on a gen or hiding near a gen will be pretty much constantly showing their aura to you.

* For Demo, I'd recommend using the add-ons that increase the length of the Undetectable status when you exit a portal. I'd recommend leaving portals near gens, but in areas out of direct line of sight from them. Being unpredictable with their placement will help. It's also a decent idea both tactically and for jump scares to put two portals near key gens. That way, even if they've seen one or both portals, they won't be able to remove them both without you knowing first. Plus, now when you teleport, you'll be able to come from multiple places and might be able to get a grab or two with the extra Undetectable time. Best case would be if the survivors only see one of the portals, because that way you might be able to approach from a different place than they were expecting.