"Well, neither of us know much about him," Frosty said.
"All we know is that he was once an Imperial Grand Admiral who led a coup against Palpatine - and against all odds, succeeded," Zuko explained. "In his 'verse, he then proceeded to decimate the Rebels, exterminate all those with Force Powers. Wanted to erase force-sensitives from the face of his 'verse - and he got his wish. Now, he rules his Empire with an iron fist, and has been known to send 'disavowed' forces to probe the various factions in this Galaxy."
"Ever since the last war with him - that was fought between the DMC, UAC, and Zaarin's Empire," Frosty said. "Ever since then, he's been poking and prodding everyone. So far, hasn't tried his hand against the UNSC, as far as I know."
"He has probed the Sangheili - twice," Zuko added. "Regardless, that's as much as anyone knows. Only one who'd know more is the Lord-Admiral himself; has a bone to pick with him. More than once, when Zaarin's boys overstep their territory, he's been there to send them right back."