Main villain of W3 was also practically main villain of W1. People forget about White Frost being shown to us in W1, how things would transpire and the fact that child of Elder blood has power either to start it, or stop it. And btw, Avalach's intentions become more clear on second playthrough, for example you can find a lot of clues in his laboratory about his interest in white frost.
So what? My point is that Eredin is just flat out bad dude trying to invade TW world. Nothing else...the game does not show anything else with him, does not try to expand his character into something more, just typical bad guy.
To compare with grandmaster he turned out to be Alvin who was told by Geralt he should use his talents and because he saw prophecies in his dreams, he decided to do it his own way. Suddenly he is more than bad dude just because you know his background and what is his motivation based on.