Why Parthia, Hatra, Adiabena exist as an army of Niger? They were independent! Where is Parthian campaigns of Severus? I think this is wrong! Excuse me for my bad english...
Yes, that states are independants, but in fact, they're allied to Pescennius against Septimus Severus. The real Parthian campaign of Severus will begin in 198 (after the fall of Clodius and Pescennius).