There are two warlock PrCs out there, one deriving power from demons and the other from devils. There is nothing in either class to prevent you from taking levels in the other.
CE Lesser drowPGtF
1/Warlock +2/Devil bloodline +1/DemonbinderDotU
1/Devil bloodline +1/Hellfire warlock +2/Fiend of possessionFF
(eldritch blast), OtherworldlyPGtF
, Practiced SpellcasterCA
(warlock), Quicken Spell-like Ability (demonbind), Sapphire SmiteMoI
, Shape Soulmeld (strongheart vest)MoIInvocations:
Baleful Utterance, Brimstone/Hellrime Blast, Eldritch GlaiveDM
Your bloodline levels add to demonbinder, hellfire warlock and fiend of possession at the same time.
You can demonbind 7/day, and binding weaker demons is a swift action. While doing so you count as Outsider (evil), allowing you to use the abilities of Fiend of Possession. Among other things, you can grant weapon enhancements totalling +9.
Your eldritch blast damage is 18d6, or 20d6 if you allow Practiced Spellcaster to increase it. 5/day you can add +2d6 with Mortalbane.
You can smite good 3/day, dealing +22 damage. If you can trade Devil Bloodline's bonus Dodge feat for Midnight Dodge then this increases to 4/day and +23.
You have DR 7/good and DR 1/cold iron.
I'm assuming Strongheart Vest works to negate the Con damage from hellfire warlock, so let's not start that argument again.
As an alternative bloodline, Djiini grants immunity to acid, emulating yugoloths for the full spectrum of Lower Plane contracts (shame there's no yugoloth bloodline).
If bloodline levels don't count against your character level, or you can buy them off, then take another three levels in demonbinder.
You could also trade Mortalbane for MindsightLoM
, since you gain telepathy 100ft while demonbound. Or pick it up via a flaw.