1/14/16:
Episode 193 is where the dementors origin was discussed. Episode 242 involved theoretical testing on how to kill a dementor (all part of leading up to this).
Also, if you're wondering why a lot of the Necromancer's new spells use English instead of Latin, it's because they were mostly invented by himself or Salazar Slytherin, who both work in English. (Hermione also had an English language spell in The Goblet of Fire that she'd invented: "Point Me," a compass spell.)