"The Dance Macabre"

So whenever I'm in a battle with Astarion in my party, and it's his turn to fight, he will often say the phrase "the dance macabre". Which I just thought was a poetic metaphor for fighting.

But then in Act 3 he completes the Necromancy of Thay and gets a spell called "Danse Macabre" which summons ghouls to fight for him.

Does that mean he just named the spell after his favourite battle phrase?

Or did he want the Necromancy of Thay because it would enable him to use the spell and he specifically wanted that, (aside from the useful info about Cazador's ritual)?

Can someone who didn't give the Necromancy of Thay to Astarion tell me what happens if Tav finishes the book, or Gale finishes the book? Is the spell you get still called Danse Macabre or is the name unique to who reads the book?