"The Immortal duel" refers to the legendary battle between Haji Valtchy and Senshi that ocurred on December 21st, 2322.B.K (Before Kosho). According to ancient documents, the battle lasted for a week and a half, and could be heard all across the universe.
While most scholars agree on the fact that Haji was the strongest of the two combatants, Senshi emerged from the battle victorious, and restored justice to the universe. Senshi trapped Haji in a separate universe called The Emptiness. The rest of the Valtchy race left Planet Earth with the decision of never coming back.
In honor of the battle, an event called Senshimas is celebrated every year on December 21st.