Harry Potter and the Goblet of Fire

[Enigma]

This is the first entry in the series where it starts off super dark and almost immediately gets to the action. Without spoiling too much, 4 murder happen right off the bat, one of which is... ehem.... mighty sus, and if I recall my experience from the first time I read it, the whole book is rather dark, and this kicks off a trend within the series where it's no longer super-happy-fun-time.