I've got far more important matters at stake!
This simply means that the speaker has many more things that he must attend to, rather than what he's currently wasting time on.
"Those days are through for me!", this signifies that the speaker finds the things he used to do, back in "those days" unworthy, and perhaps possesses a lower priority to him, as of now. So he has stopped doing them. Let me explain the context with an example.
Consider a contract killer, who has gained popularity due to his brutal ways, but has left his ways to take personal revenge on the ruthless king. One day, he gets an assignment to kill a unhealthy man (he used to take up any jobs for a reward, back when he was a young man). Now he tells the person who comes with the contract :
"Those days are through for me! I've got far more important matters at stake!"
This means that the killer has left his ways of killing for money and finds it unworthy of his time and energy. He has moved onto something more important, which when compared to his contracts from the past, means next to nothing.
PS : I've used a story line-based example because you have quoted this from "Warcraft", and you may relate to the answer better.