Brothers is the 8th optional quest. It is acquired by talking to Izlude in Kalm after completing the quest Kill Elmdor. The objective of the quest is to fight and defeat the Brothers, Sacred and Minotaur, and return to Izlude afterwards. You have to fight Sacred and Minotaur at the same time and you have to defeat both of them. Upon returning to Izlude, he rewards you with 2000 EXP and 2000 Gold, the quest is complete and you can now obtain the quest Slay Kilrog.
Also, 5 minutes after defeating the Brothers, you can obtain the quest Eidolon Challenge by talking to them. However, if you have not talked to Izlude to complete the quest yet, the game instead just waits another 2 minutes and then checks again.
In this engagement Minotaur deals 143-153 (+80) normal damage having 7(+39) heavy armor and 4150 base HP, plus 1700 HP from equipment and 1270 MP total. His attributes are 126 Strength and 58 Dexterity and Intelligence. Not to be fooled by the Strength focus Minotaur reaches extreme swinging speeds with his Giant Axe having Attackspeed Plus, Endurance Aura (not affecting Sacred), Move Plus and Rage (like the Ninjas Battle Rage). Besides the aforementioned Giant Axe he also wields a Mithril Shield, Diamond Helmet, Mithril Mail, Defense Ring and can fall back on 5 X-Potions. It is recommended to engage him as the later of the two as his block chance and damage reduction make it way harder to finish him then his brother.
In this engagement Sacred deals 99-113 (+57) normal damage having 3 (+19.2) heavy armor and 2975 base HP, plus 800 HP from equipment and 725 MP total. His attributes are 87+2 Strength, 31+2 Dexterity and 29+2 Intelligence. Compared to his Brother Minotaur, Sacred could easily be called a pushover having nowhere the damageoutput or the survivabilty with only 18% chance to block granted by his Aegis Shield unlike the Mithril Shields 25% chance. The remainder of his items and his abilities are also inferior. His abilities are Move Plus and a Pulverize ability which seems to trigger 100% of the time. His weapon of choice is a Mithril Axe and while also being clad in a Mithril Mail he only wears an Iron Helmet and a Ruby Necklace. The strong point - one could say - is that he too has access to 5 X-Potions.