The Hellfire Warlock text says:
At each level, you gain new invocations known, increased damage with eldritch blast, and an increase in invoker level as if you had also gained a level in the warlock class. You do not, however, gain any other benefit a character of that class would have gained.
I have a Rogue 4/ Warlock 8/ Hellfire Warlock 3 so for my eldritch blast damage I consider myself like a Warlock 11 (6d6).
On my next level I plan to take a level of Warlock, but the feature of the 9th of Warlock is just increase of eldritch blast damage.
Do I increase the damage of my eldritch blast again (maybe a little overpowered) or is the 9th level of Warlock in my build lost (no gain)?
Best Answer
You are correct - you would not gain +1d6 Eldritch Blast
As you yourself wrote, you are effectively a Warlock 11 for Eldritch Blast damage, and going up to effectively Warlock 12 does not increase it.
But look on the bright side - your Fortitude and Reflex saves go up by 1!
If you do not think you will get to warlock level 10 (effective level 13), consider multiclassing instead of taking warlock 9. Just as an example, rogue will add damage to your blast if you can catch opponents off-guard, and fighter will let you pick up feats like Weapon Focus (rays). If this is something that interests you, I recommend creating an additional question.