From Warhammer 40k - Lexicanum
Rhetoricus was a member of the Imperial Fists Chapter.
He was a highly-regarded scholar, considered second only to the Imperial Fists' Primarch, Rogal Dorn, when it came to battle strategy. He codified the Chapter's modern approach to battle, penned across a number of tomes. The most notable of these was The Book of Five Spheres.