Originally Posted by
Caerule
Basically, the Void Lords want to consume all matter and energy of the physical universe. However, as void creatures, it would take incredible effort and energy for them to manifest beyond the Void. So their ability to do stuff is severely limited.
Titans are born from worlds with souls. Few worlds have a World Soul like this, but making life thrive on them is a way to stimulate and awaken them. The how of that is unclear, but perhaps the souls and natures of its inhabitants effect and evolve it in some way.
Unlike the Void Lords, the Titans have tremendous ability to interact and shape the physical universe. Something the Void Lords envy. Their solution was to create the Old Gods and hurl them out into space, to impact planets around the universe and convert them to hellish twisted shadow worlds. With the hope that one of these worlds would contain a World Soul, and would arise as a Dark Titan: A Titan twisted by the Void that could basically hand the universe to them on a silver platter.
Sargeras was driven mad when he found a slumbering tainted Titan. Realizing the plan of the Void Lords, he believed the only way to prevent this plan from succeeding is to kill all worlds. He freed the demons to be his army towards this goal. Unexpectedly releasing the Fel that had bottled up in their prison, forever tainting himself into a demonic Titan. A monster of evil and cruelty, that set about destroying the universe.