Essentially, was the explosion a means to an end or simply an unfortunate side affect? Just making an explosion in an area with thousands of innocents and the presence of a huge amount of lyrium (remember Oghran's comments in DAO) would be sufficient to rip open the Veil. This is effectively what the Magisters did do in the past, so in effect it was recreating their ritual.
So the intent could just have been to destroy the peace conference and the side effect was the Veil tear but I must admit that now seems unlikely, with the voice over of the Elder One. His followers would have known the deaths and the lyrium would be sufficient to achieve their purpose, if this was opening the tear, but the choice of location was deliberate to maximise the demoralising and chaotic effects of their action. Of course the people who planted the bomb (or whatever did cause the explosion) may not have been aware of who they were working for. Many of the events currently occurring Thedas are apparently being orchestrated by the Elder One and his followers without them necessarily being aware of that fact. The general theme seems to be to maximise chaos and so this would seem to be what is required to aid the return of the Elder One. Bloodshed and strong negative emotions have always been associated with a weakening of the Veil. People tend to react badly to situations outside of their control (chaos), so the greater the chaos, the more negative their reaction is going to be.
Whether or not the Black City is involved with the story, I'm hazarding a guess the Elder One will be some form of Chaos Demon who can only truly exist in the Fade and will return when the breach is complete and the material world and the Fade merge into one.