What do You think happened to the Indus Valley Cities of Mohenjo-Daro and Harappa? Do You think they were slaughtered by invaders or the Civilization died out from some kind of dramatic climate change or happening(Like terrible drought,Disease or other thing)?

Well, we do know that another people called the Aryans came into the area as a large group, so it is possible they brought a virus with them, much as the Europeans did when they went across the Atlantic in the 1500s.

Prolonged drought due to climate change

i think prolonged drought forced them to disperse into the surrounding cultures much like the south american civilisations