Every faction in the tower has a unique ghost skin that is rewarded to you once you have reached rank 30 in that faction. The known ghost skins are as followed
Vangaurd (Orange and white V ghost)
FWC (Blue and red warstripe ghost)
New M (Red and white striped ghost)
Dead O (Back and white speckled ghost)
Queens Emmisary (pink and yellow sigil ghost)
Iron banner (Silver plated emerald eye ghost)
Crucible (Rust red and scratched ghost with red eye)
Cryptarch (unknown)
Shipwright (unknown. Rewarded with ghost schematic)
Xur (stealth black ghost available in November)
Now get out there and grind like the wind!!!
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Cryptarch is certainly wrong, I'm passed level 30.... I did get a nice dye module but no ghost shell