Well then, lets see about your class. First, I'll note I quite like the concept. It's always fun to see a dancing based class, and necromancy is one of my favourite schools of magic.
Skills seem fine, they match the theme of the class.
The spellcasting progression makes me a bit wary, but seeing as you need levels in a different class from the one being advanced (even if it is a class with minor progression of its own), it may be fine.
Bardic music usage advancement fits fine.
Disciplined Mind is a nice flavourful ability.
Sense the Opening Door is an interesting ability. Given the flavour of it, it may also be fitting to at some point grant an ability to easily identify creatures as living, non-living, dead, or undead.
deathwatch spell while dancing.
Last Dance is an interesting capstone, and while powerful, doesn't seem problematic given its ability to only be used successfully once.
On Age Draining Waltz, I'm not sure the use limit on specific creatures is needed, given the time required to pull it off. Then again, I may be slightly biased with regard to that, as the limit prevents the interesting idea of a Dancer dancing with someone as they gradually get younger, while the other person gradually gets older, until the ageing person eventually dies.
Only skimmed the rest of the dances, though I'l try to look them over in more detail later today. That said, it might be worth adding a feat that allows you to learn additional dances you qualify for, possibly with the prerequisite of already knowing 9 dances, or noting dances learned by the feat don't count for learning other dances. This was a Dancer may continue advancing their dances after reaching level 10.