In the HORUS HERESY 1, Visions of War, page 5 is part of a wonderful Adrian Smith picture, depicting the Emperor confronting Horus.
but who are the warriors behind the Emperor? are they marines or custodes or what? as far as i remember the Emperor only brought Dorn + Fists, Sanguinius + Blood Angels and a small detatchment of custodes..
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Good question about those guys. The 'official' answer is that they are Custodes, and Adrian Smith painted them silver/gunmetal so they wouldn't detract from the Emperor in gold.
However, I prefer to think that they are the last of the Thunder Warriors (see the red eyes!!!). They are in antiquated armour, with the Emperor's own symbol on their shoulder shields. The Outcast Dead also suggests that the remaining Thunder Warriors are still loyal to Him, despite being 'discarded'.