Kushadhwaja (also known as Kushadbhojan), was younger brother of Janaka. Janaka's daughter Sita was wife of Rama, the leading character of Hindu epic Ramayana. Kushadhwaja's two daughters Mandavi and Shrutakirti were married to Rama's younger brothers Bharata and Shatrughna respectively.
King Kushadhwaja had his seat in around Rajbiraj, where there is still an old historical temple of Rajdevi Temple with more than thousands year old idol lies of various Hindu Gods and goddesses. Temple along with chinnamasta temple is key temple of Maithali people. Around Rajbiraj also lied shrines dedicated to his daughters Mandavi and Shrutakirti who were married to Rama's younger brothers.
Kushadhwaja later moved to Sankasya, also known as Sudhanva, after his brother won a war against Sankasya, when Janak was attacked by the ruler of Sankasya. He continued to rule the area around Rajbiraj to his through key ministers as per local tale.