Must have been thinking of something that applied both Cowering and Helpless simultaneously... Of course, even if it worked at all, it wouldn't work on any creature immune to either critical hits OR fear, which can easily include a substantial proportion of foes.
Is Nightmare Spinner the class that allows you to pierce Mind-Affecting/Fear Immunity with spells? Try to get the DM to allow you to do that with your Intimidate skill as well.
Finally, if you use Mearls' changes to the Hexblade, then having a DC of 15+Charisma mod will be fine since you pretty much net infinite retries until you land it.
You might be thinking of dreadwitch in heroes of horror at 4th lvl you can affect things normally immune to fear also alot of other goods like class lvlsx2 to intimidate and boosted CL while under a fear effect yourself
I looked at Dreadwitch first but it lacks one of the spells known to qualify. If you could get it you could get your familiar Never Outnumbered and intimidate you too Weird that you're better at scaring people when you're shitting your pants yourself.
Not really, $10 says this is scarier than her "normal" face:
http://images.cheezburger.com/completestore/2010/8/5/83cb0bd2-69c5-450a-911c-903bfa82d1bc.jpgAs for scare not being on the Hexblade spell list... that seems like an oversight, or an omission of "it's not actually useful at the level they learn it, so we just won't bother putting it there." They have both Cause Fear and Fear, which are the immediately higher- and lower-level spells in that little sequence. It's a terrible spell that you're never, EVER going to cast, anyway, you just have to pay a tax on your spells known at 8th-level, which just so happens to be the best point to bail on the base class after the Mearls enhancement. After that, I suggest taking Dread Witch 4 (if the DM allows), and then taking or leaving the last level depending on whether or not you want what it has to offer. You can now pierce immunity to fear about 3 levels before it becomes incredibly widespread.
The next big PrC is Abjurant Champion. The only thing that you're not gaining from Abjurant Champion are the AC bonuses, so I'd think that finding some way to stick Shield and/or (Greater) Mage Armor on your spell list would take high priority once you've gotten your core shtick out of the way. Your 12th- and maybe 15th-level feats are available for this.
That leaves you with Hexblade 10/Dread Witch 5/Abjurant Champion 5. If you want, you can drop 1 level of Hexblade for Mindbender and get Mindsight, which is all kinds of useful. Drop another for a +1 BAB/+1 Casting PrC and you might pick up some other useful oddities, but you really want both the +1 BAB AND the +1 Caster level, because the BAB increases your CL and the 19th caster level gives a ton of spells per day (relative to the overall size of your pool, at least).