The apprentice who kills Plagueis that story is referring to is meant to be Palpatine, Darth Sidious. But before Sidious, the idea appears to be that during his artificially extended life ...
"Give in to your anger," Darth Sidious said to Luke Skywalker ... but his all-around game is what makes him so appealing, different from Zion, whose power, strength and fluidity made him so ...