Eidolon Challenge is the 32nd optional quest. It can be acquired by talking to Minotaur some time after completing the quest Brothers. The objective of the quest is to find the four "Friends of Brothers" and defeat them in battle. They can be encountered in any order but are not marked on the minimap until found.
After defeating all four Friends of Brothers, the player may return to the Brothers and is rewarded with 4000 EXP, 4500 Gold and Quake Materia. This also unlocks the quest Yet Another Try which can be obtained by talking to Minotaur.