True, so with draegloth who is level 5 as warlock needs one more level, then 3-4 levels and before hitting epic levels he gets one PrC level. Overall greater buck for power would be going for hellfire warlock which is 3 level PrC and you can qualify for it in level 6 or 7. But as said being warlock is just a option and not must. But overall playing caster as draegloth means you never reach higher end spells and stuff so best to play from beginning as hybrid of melee class plus something else. Or push for martial prowess by multiclassing into martial favored classes and PrC's. Draegloth assassin could be quite curious combo but could work (by taking feat which adds some more skills into outsider list). Pure barbarian levels are not bad investment either. Or go for fighter levels and barbarian level and then spiked chain or some other big large sized weapon with a reach.