The Destiny database site has a few class items related to Osiris. Nothing on the Seraphs.
I'm inclined to believe that Seven Seraphs is only accessed very late game, or was cut entirely.
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If you look in crucible, each mode is associated to a faction. Control is dead orbit i believe. There were some that had a symbol with a 7 on it soo im guessing 7 seraphs is in the game
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SAD FACE