What is Bolas' actual plan[Vorthos]

So, in story, Bolas unsundered Alara and absorbed mana from the Maelstrom, then he unleashed the Scorpion, Locust and Scarab God to help raise an army of Eternals and he's got Ral Zarek and Tezzeret working on building a new Planar Bridge.

So why is Bolas building an army? Just for generic conquest? And why did Bolas send Tezzeret to Mirrodin to spy on the Phyrexians? How do they play into the plan?

I feel like I'm missing things here, were they just not revealed yet, or am I an idiot?