Warlock actually feels quite powerful. They make an incredible party face due to innate CHA focus, and the Imp summon can both fly and has invisibility, making it the best scout in the entire game.
How to build a Warlock centers on their main cantrip, Eldritch blast. Just like in tabletop, you will cast this every single turn. There are two perks that improve this cantrip, one for damage, one for knockback. Taking both of these is very worthwhile as a powerful baseline for your character.
The other main spell you want to take is Hex. Hex improves your damage of your Eldritch Blast and has two important things that make it efficient. The first is that casting it is a bonus action, not a regular action, so you Hex your target and simply blast them. The second it that you can re-cast Hex for free as long as you don't lose concentration. So for this single spell slot you are getting an enormous amount of damage.
The nice thing about Warlock is that since damage is already covered by such a powerful cantrip, your other spells don't need to be damage-related at all. I really recommend Misty Step and Invisibility. Misty Step lets you escape or instantly position yourself on high ground, while Invisibility has incredible utility out of combat.
