EzekielRaiden
Follower of the Way
Ultimate Spylock
A build which works with almost any patron or playstyle (though GOO and Fiend offer secondary benefits that can help).
Origin feat: Any, though Lucky and Skilled are probably best
4th level feat: Actor
Required Invocations: Devil's Sight (Warlock 2+), Mask of Many Faces (2+), Master of Myriad Forms (5+), One with Shadows (5+)
Required non-class Skills: Stealth, Persuasion, and (potentially) Performance
The earliest you can get all of these features is level 5. At that point, you'd have 1 free invocation (I recommend Agonizing Blast), and you are now the single best infiltrator in the entire game. As long as you stay in dim light or darkness, you can turn yourself invisible (One with Shadows), and you can see perfectly fine out to 120 feet in all forms of less-than-bright-light (Devil's Sight). While still invisible, you can change your clothing (Mask of Many Faces) to resemble whatever target you desire, and you can drop your invisibility to alter self so that you physically resemble that target (Master of Myriad Forms). Then, if anyone actually approaches you, Actor allows you to imitate the target's voice as well. The only possible failure-point is someone touching your clothes and realizing they're illusory, which can potentially be averted if you're able to get a costume first. (Note that disguise self does not require concentration, so you can in fact have both effects active at the same time.)
Get in, learn what you need to learn, get out. If second-story work is required, that one free invocation could instead be used for Ascendant Step, allowing you to levitate effectively at-will (albeit not while you are invisible, unfortunately).
Actor also gives you Advantage on your checks to pretend to be someone you aren't. The Fiend Patron's Devil's Own Luck feature helps there, and with any Stealth rolls you need to make to avoid being heard while invisible. Meanwhile, the GOO patron makes your disguise self silent, and at higher levels gives you modify memory so you can potentially even stash an unconscious person somewhere, impersonate them, and then modify their memory so no one will realize they've been impersonated.
Even if you want to do other Warlock things (like Blade Pact), you can sprinkle this in across levels, growing from an amazing scout to an incredible spy/infiltrator. Since it only requires 4 Invocations and a single feat, and is ultimately compatible with any patron, you can almost certainly mix this in with any other playstyle and build. If trying to be a Blade user, for example, you can have the scout-y core online by level 5 (in order: Blade Pact, Agonizing Blast, Devil's Sight, Thirsting Blade, One with Shadows), and add the remaining "spy" elements later, possibly allowing you to take Cha +2 at level 4 and save Actor for level 8.
A build which works with almost any patron or playstyle (though GOO and Fiend offer secondary benefits that can help).
Origin feat: Any, though Lucky and Skilled are probably best
4th level feat: Actor
Required Invocations: Devil's Sight (Warlock 2+), Mask of Many Faces (2+), Master of Myriad Forms (5+), One with Shadows (5+)
Required non-class Skills: Stealth, Persuasion, and (potentially) Performance
The earliest you can get all of these features is level 5. At that point, you'd have 1 free invocation (I recommend Agonizing Blast), and you are now the single best infiltrator in the entire game. As long as you stay in dim light or darkness, you can turn yourself invisible (One with Shadows), and you can see perfectly fine out to 120 feet in all forms of less-than-bright-light (Devil's Sight). While still invisible, you can change your clothing (Mask of Many Faces) to resemble whatever target you desire, and you can drop your invisibility to alter self so that you physically resemble that target (Master of Myriad Forms). Then, if anyone actually approaches you, Actor allows you to imitate the target's voice as well. The only possible failure-point is someone touching your clothes and realizing they're illusory, which can potentially be averted if you're able to get a costume first. (Note that disguise self does not require concentration, so you can in fact have both effects active at the same time.)
Get in, learn what you need to learn, get out. If second-story work is required, that one free invocation could instead be used for Ascendant Step, allowing you to levitate effectively at-will (albeit not while you are invisible, unfortunately).
Actor also gives you Advantage on your checks to pretend to be someone you aren't. The Fiend Patron's Devil's Own Luck feature helps there, and with any Stealth rolls you need to make to avoid being heard while invisible. Meanwhile, the GOO patron makes your disguise self silent, and at higher levels gives you modify memory so you can potentially even stash an unconscious person somewhere, impersonate them, and then modify their memory so no one will realize they've been impersonated.
Even if you want to do other Warlock things (like Blade Pact), you can sprinkle this in across levels, growing from an amazing scout to an incredible spy/infiltrator. Since it only requires 4 Invocations and a single feat, and is ultimately compatible with any patron, you can almost certainly mix this in with any other playstyle and build. If trying to be a Blade user, for example, you can have the scout-y core online by level 5 (in order: Blade Pact, Agonizing Blast, Devil's Sight, Thirsting Blade, One with Shadows), and add the remaining "spy" elements later, possibly allowing you to take Cha +2 at level 4 and save Actor for level 8.
