In The Avengers: Infinity War, Thanos wants the Infinity Gems because

having all of them allows him to destroy half the population of the universe with a snap of his fingers.

At least a few other characters know about this, and also that his plan is to do exactly that.

Although having all six gems understandably grants a person an unbelievable amount of power, this incredible ability might have just been hyperbole. But at the end,

Thanos does exactly this and kills half the universe.

How did he, or anyone, know that he would have this specific capability?

In Avengers: Infinity War, Thanos wants the Infinity Gems because

having all of them allows him to destroy half the population of the universe with a snap of his fingers.

At least a few other characters know about this, and also that his plan is to do exactly that.

Although having all six gems understandably grants a person an unbelievable amount of power, this incredible ability might have just been hyperbole. But at the end,

Thanos does exactly this and kills half the universe.

How did he, or anyone, know that he would have this specific capability?