Spoiler (click to reveal)
You open up the third villain location. Voldemort is added to the villain deck. He is always placed at the bottom of the deck face up, so when the second to last villain is flipped, V's card is revealed at the same time. You thus have four active villains (three slots plus Voledmort.) You cannot attack V until the other three are gone. I don't know what his text card reads, but the whole setup is obviously going to be a major game changer. Unless his villain card is replaced with a different version in later years, I'm guessing you can't play years six or seven without that card. It seems as though it is critical to setup and gameplay.
I've only briefly glanced at the Hogwarts cards, but the year five rulebook does not mention any rule changes here aside from adding them to the existing deck. From what I've seen, its more of the similar in this respect - new spells/items/allies to buy, but no new mechanics.