Lower brachial plexus lesions involve C8 and T1. They are the result of traction on the arm in an abducted position - e.g. during a difficult labour; or at birth - Dejerine Klumpke's palsy.
The patient presents with a claw hand - the result of paralysis of the intrinsic hand muscles; sensory loss along the ulnar side of the hand and the forearm (first thoracic dermatome); and if the T1 root is affected, possibly, a Horner's syndrome from interruption of sympathetic innervation at that level.
Note that a combined ulnar and median nerve lesion produces a similar picture in the hand but with additional involvement of flexor carpi radialis and pronator teres.