Head devoter of Mahabharata

Yes, there was a head devoter in Mahabharata, he was Barbarik, the son of Ghatochkach and grandson of Bheemsen.

Hidimba, his grandmother had gave him a knowledge of warfare. Later on he met with  Vijay siddasen ( the priest of Kamaxya Mata) and Barbarik asked him the way to be powerful and strong. After that Vijaysiddasen suggested him to do (meditation or contemplation) to asking help to Kamaxya Mata. It was very hard meditation and took many days. He prayed to god successfully and got 3 arrows and sindoor( powdered cinnabar). Through that 3 arrows he could win any war or battle that he involved. Even Mahabharata could finished by himself only with that arrows.

By happily he wanted to give Vijaysiddasen a guru Dakshina but instead of Payment he took promise from Barbarik to use this weapon from weaker side of the war. That means if he fight from Pandavas against Kaurav, he will change the parties towards Kaurav if they become weak. 

One day Barbarik met with 5 pandavas and shri krishna at their palace. He showed interest to take part in war from first day with explaining his promise to Vijaysiddasen.

Shri Krishna thought 🤔 if he took part in Mahabharata war, the war will not end  because he can subsequently change the side if Pandavas become weaker than Kauravas. So Shri Krishna insist him not to take part in war but Barbarik didn't agreed with that. Then Shri Krishna wanted his Guru Dakshina from Barbarik because once he assumed Krishna as his teacher as recommended by Hidimba( Barbarik's grandmother). Barbarik was ready for it. After that Shri Krishna said, " As a Gurudakshina give me your head ". Barbarik cut his head and devoted to Shri Krishna. Barbarik wanted to see the Mahabharata war so Krishna had put his head at the top nearby Kurukshetra.  He is the only one who saw Mahabharata war from the first day.

Nowadays Barbarik is worshipped as a Khatu Shyam Ji / Khatu Shyam Baba.