@Azel, I'm putting together a level up plan for the Myrkdreki Twins and don't see where they could be picking up a feat yet. They'll only be going to 20 HD. Did you want them to retrain something?
What did you have in mind for their 6th level spell? I know you weren't too enthusiastic about more Shadow spells for them, but Greater Shadow Enchantment is pretty nifty and doesn't have the same disbelief issue as most of the other Shadow spells. On top of being able to mimic such great buff spells as Heroism and Grand Destiny, it's also got goodies like Feeblemind and Dominate Person. It's hard to go wrong with Disintegrate, too.
If you want to stick with Necromancy,
Fleshshiver or
Imperious Glare are both good choices. Imperious Glare has the benefit of being able to target multiple creatures and has a higher DC due to the Myrkdreki's Frightful Presence, but it's also a Mind-Affecting Fear effect, meaning we're going to run into more creatures who are immune or at least resistant to the effect. Fleshshiver is single target, but auto-Stuns anything with HD equal to or less than caster level, forces a save to avoid being Stunned if their HD is higher, and kicks in for damage and Nausea in the following round.