EX Champions' Ballad is a main quest from The Legend of Zelda: Breath of the Wild. It is unlocked upon installation of the DLC Pack "The Champions' Ballad". After freeing all the Divine Beasts, Link receives a message from Zelda requesting to return to the Shrine of Resurrection. Here he obtains the One-Hit Obliterator, and is tasked with a quest to defeat four sets of enemies with it. For each group of enemies defeated, a new Shrine will open up. Upon completing all four Shrines, the One-Hit Obliterator will burst into four different-coloured orbs that soar through the sky to land relatively near the towns of each Champion.
On each of these four locations, Link meets Kass and learns more about the unfinished The Champions' Ballad song his teacher left behind regarding the four Champions. The four main quest "EX Champion Daruk's, Mipha's, Revali's and Urbosa's Songs" are triggered. Each part of the song contains three trials the respective Champion also had to overcome. Upon completing all three of a champion's trials, Link is summoned to the respective Divine Beast. From here, he enters the Illusory Realm to fight the bosses once again, only everything Link can use will be provided. These items are usually barely enough to defeat the boss. By defeating the elemental-blight Ganon scourges, the respective power will have its recharge-time cut by 2/3rd.
When Link has completed all four Champions' songs, Link will be requested to head back to the Shrine of Resurrection. As soon as the Sheikah Slate is placed back in its pedestal, the entire chamber starts to descent, leading Link to the Final Trial. It works just like any other Divine Beast dungeon, and upon completing it and defeating the final boss Monk Maz Koshia, Link is rewarded with the final Rune, the Master Cycle Zero, and the Champions' Ballad song will be completed.