Malygos was the leader of the Blue Dragonflight, husband of Sindragosa. Malygos went insane and lunatic after the decision of his younger sister Alexstrasza to work together with the humans in order to protect Northrend. Denied with this decision and disorder, Malygos started to become tired and weak and went to the Nexus where he hide for decades.
Many Heroes entered the Nexus and tried to kill him or convice him to cooperate with his sister but unfortenatelly all died. It was then when Alexstrasza sent Drumans and the rest of AfterHours to slay him since the blue dragonflight was destroying big part of the nature of the lands with magic.
When Malygos face Drumans, he knew that Alexstrasza sent him and without further questions or talk they started a fight. At the last minutes, malygos destroyed the entire nexus plattform powering up to a battle level equal to the lich king and started blasting drumans and a raid of red dragons.
At the end Malygos couldn't resist and fall to the infinite void.