In greek mythology, who was given knowledge of the future but impotent to stop it?

Question

He was given knowledge of the future by the gods, but impotent to do anything about?

Answer 1

Kassandra

"According to one version of the story, Cassandra received the power to foretell the future from the god Apollo. Apparently, Apollo instructed the mortal woman and taught her about the art of prophecy because he had an ulterior motive - the god wished to win her affections. Cassandra accepted Apollo as a teacher, but not as a lover. Naturally, the god was insulted by this refusal. So he punished Cassandra. Apollo caused the gift that he gave Cassandra to be twisted, making everyone who heard her true and accurate foretellings of future events believe that they were instead hearing lies. In other words, the wondrous blessing bestowed upon a mortal became instead a terrible curse. "

She predicted things such as the Trojan war, and the defeat in it.  Above is simply one version of the legend.  There are many, many more.

Answer 2

Cassandra possibly?