NV2 (Apprentice) is the second rank in the Dark Brotherhood, which is awarded after participating in a Competition as an NV1 (Apprentice).
It is part of the Novitiate class, the second step down the path to NV4 (Proselyte/Trainee/Trooper).
The requirements for the promotion from NV2 are found at the above link, the completion of the above requirements will earn a promotion to NV3 (Novice/Recruit/Cadet).