All warlock or multiclass sorcerer?

[bid]

I'm playing in out of the abyss so I'm not sure if I will be fighting a lot of humanoids to make my specter. My DM said it's going to level 15, so I thought warlock 5 sorcerer 10. How important is lifedrinker?

I also think I'm going to take great weapon master. quickening true strike will help me add a lot of damage.

You can't use Cha with GWM, all heavy are 2-handers. Was that changed in Xan?

Lifedrinker and GWM run in opposite ways. If you use GWM, lifedrinker becomes weaker, twice since you don't boost to Cha20. With GWM and lifedrinker, you remove 5 chances of doing weapon+10 to do weapon+20.

And you'd take GWM at lock 5 / sorcerer 4. Compare to lock 9 with GWM and +2 weapon. Quick maths, assuming Cha18 GWM:
- lock 5 GWM, +1 weapon = 2d6+15 * .45 ~ 9.9 per attack
- lock 12 GWM, +2 weapon = 2d6+22 * .50 ~ 14.5 per attack
After turn 3, you'll have done 59.4 with sorcerer vs 58 with lifedrinker.

You don't lose much combat-wise, and sorcerer will help you out of combat.