Before the War of Wrath, Manwë Súlimo King of Arda, Lord of the Valar, and caretaker of Valinor Untouched and Melkor were brothers and the most powerful but wanted different things in life, like Melkor weaved Evil into Arda and Manwe sought peace and commerce and even walked amongst the Elven species and held conversation with them (most famously his Speech to Fëanor about restoring Yavanna's Trees). He was refused the Silmarils by Fëanor because Fëanor loved them too dearly. After Manwë and the other Valar kicked Melkor out into the Void forever he vowed to remain in his throne until the last day of Doom when Melkor will destroy the Door of Night,Sun and the Moon, Rescue Ar-Pharazôn from the Caves of the forgotten and be challenged by Turin Turambar and killed by him.
What's the reason behind this?
It seems rather odd that he would commit to an act of righteous commitment until the end of the world. Is it because Melkor was his brother? And he wanted to bide his time by taking a vow of paralyzation until the Time had come for Dagor Dagorath?