buddy there might be different version in other cultures regarding the originatio of deusi, but as per kirati mundhum it is as follows,

Origin of Tihar festival:



The tihar festival came into vogue to commemorate the good name of
the Kirat king, Balihang. The king Balihang was sick. When he became
seriously ill, all his subjects were very sad. Seeing the gloomy faces
of his subjects the king told them to pray to almighty if they wanted
his recovery. All the subjects prayed to the almighty for their king's
recovery.



Fortunately, the king became well. So they all lit lamps making the
dark night bright and took parts in the rejoicing. The king Balihang
sent the youth through out his kingdom to spread the good news of his
recovery. As there was no modern medium of communication, the youth went
house to house singing and dancing.



As there was no modern medium of communication, the youth went
house to house singing, dancing and spreading the news. Thus a tradition
of Bhaileni and Deusi Bhailo was commemorated every year.



It is proven by historical and cultural practices in Nepal: That's
why you never see Tihar & Deusi Bhailo in India which Hinduism is
supposed to have started. This festival, Tihar & Deusi Bhailo
belongs exclusive to Nepal.



Even today they say on the deosee-ray’s song, "we did not come on
our own, on the command of the king Balihang, we have come here".



When the youth returned to their own homes, they celebrated and
rejoicing the sisters gave tika to their brothers. After that night (new
moon) the Mongol, Naywars celebrate their New Year.