Long time ago when magic and sword do exists, the world was divided into kingdoms ruled by kings who drew power from gods of Olympus through prayers and beliefs.
When believers made the gods happy through their devotion, the gods would offer them their blessings.
Not only kings but also other humans sought their power through devotion to the gods. The gods bestowed blessings upon those who showed loyalty and reverence.
In those times, demons were always mistreated. Humans misunderstood them as foul creatures who wanted to harm them. That is why they sought power from gods to defeat them.
There was always a fight between demons and humans. There was a law of nature that no god and demon god could interfere directly with humans. However, the Gods of mount Olympus interfered with this very law of nature and indirectly found a loophole to give power to humans in the form of blessings. The role of the heavens was only to provide a better place for humans after death, manage the reincarnation of souls to a better next life, and role of hell was to deal with the foul and corrupt humans in hell as punishment after death
Zeus has always worried about Hades because he thinks that Hades will overthrow him from Olympus but in reality Hades always love and respect Zeus as elder brother. Hades always takes his responsibility seriously to keep the dead in the underworld but Zeus never realized because he was blinded by the throne of Olympus. When Zeus realized that humans couldn't defeat the demons with the power that they got in form of blessing, his jealousy blinded him, and he decided to interfere with the law of nature. He directly attacked hell to eradicate all the demons and Hades. Zeus marched with the army of Olympus in hell and the battle raged for fourteen nights and days, but ultimately, Zeus suffered defeat. Fearing the loss of Olympus throne, he proposed a peace treaty. Hades, as the king of hell, did not desire war and the loss of innocent souls, so he agreed to sign the treaty.