Jalandhar was Shiv ansh himself and could be killed by Lord Shiva only. In childhood itself Jalandhar had the power to fight Lions.Upon growing up he was made the king of demons by Shukracharya.He could not be killed easily since his wife was very pious so Vishnu had to deceive his wife and spend a night with her then only Jullundher could be killed .Upon his death his soul merged with Lord Shiva.
