Lord Sanguinar is one of the four blood elf advisors that aid Kael'thas during the boss encounter in the Eye. He appears to be a paladin. Kael'thas refers to him by the title "The Blood Hammer".
See Kael'thas Sunstrider (tactics) for details on the fight.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials.*
He and Valeera Sanguinar may be related.
His name is derived from the Latin word sanguine, meaning "blood".