My group uses the standard array with an additional +2 bonus for uber munchkiny goodness.
Otherwise it's all the usual rules. I chose the great old one patron and the pact of the tome, for the cantrips, the rituals and the subtle spells on the list.
His basic schtick is to infiltrate, while maintaining contact with the party through his familiar (Book of ancient secrets), short-ranged telepathy (awakened mind) and cantrips (Message). He can then apply an assassination attack when combat goes down (all hits are crits with surprise!)
Please tell me what you think.
Warlock Build: - Assumes standard array +2
Var. Human (Spell Sniper Feat)
Charlatan background
STR: 10
DEX: 14 +1 = 15
CON: 13
INT: 12
WIS: 8
CHA: 15 +2 + 1 = 18
Skills: Persuasion, Stealth, Perception, Acrobatics, Athletics, Deception, Slight of Hand
Forgery kit, Disguise Kit, Thief’s Tools
01) Rogue 1 Expertise (stealth & theive's tools), Sneak attack 1d6
02) Rogue 2 Cunning Action
03) Rogue 3 Assassinate, Herbalism Kit, Poisoner’s Kit, Sneak attack 2d6
04) Warlock 1 Awakened Mind
05) Warlock 2 Agonizing Blast & Mask of Many Faces
06) Warlock 3 Pact Boon (3 Cantrips)
07) Warlock 4 +2 Charisma
08) Warlock 5 Devil’s Sight
09) Warlock 6 Entropic Ward
10) Warlock 7 Book of Ancient Secrets
11) Warlock 8 +1 Dex & Con
12) Warlock 9 Repelling Blast
13) Warlock 10 Thought Shield
14) Warlock 11 Mystic Arcanum (6th)
15) Warlock 12 +2 Dex Misty Visions
Thanks for your feedback.
Personally, since I'm considering something very similar, I dig it.
Yours is probably more optimal than mine. I'm choosing the Swashbuckler rogue path instead of Assassin, to serve more of a skirmisher with spells type role. But, like you, I plan to choose Great Old One and pact of the tome. Even the progression is similar to how I see it working for my character. At level 5, I actually planned on grabbing Book of Ancient Secrets (you have it at 10) and Misty Visions (you have it at 15).
Just keep in mind you don't get sneak attack damage with spells. Also, I would be a little concerned with such a bad perception score if you are going to be off by yourself, you might want Expertise in it.
Coredump00 wrote:
Why not go the other way round Warlock 3/Rogue X
You still get Mask of Many Faces + loads of useful cantrips and some decent spells and Awakened Mind
Swap Dex and Cha and take a ranged weapon option
Your Sneak damage increaces dramatically which all doubles on your Assassin Crits
I know some of the Assassin features seem a bit situational after 3rd, but the Rogues are superb
Uncanny Dodge, More Expertise, Evasion, Reliable etc.
Expertise in Deception with MOMF with Actor Feat with Friends, even you don't know who you are anymore.
Or go bladelock 5 thirsting blade to give that rogue 2 base attacks... 
Or if you switch to Archfey you could go 7 for Greater Invisibility.