Manishtu is the ancient Shardmind Pog Middlethorp was so bent on saving on the day of Mourning. A psion of unprecedented power in his youth, Manishtu has long since retired from adventuring and now spends his days studying the subtle movements of stars and nations, hoping to uncover a pattern in the Draconic prophecy.
He has become a sort of patron to the party, offering cryptic bits of wisdom from time to time, but on the whole his motives and actions are not entirely clear. What is clear is that no other mortal being understands as much about the prophecy as he does, and at the very least, he is pushing events towards some point. The party is no doubt part of his machinations.
He is old, very old, and likely near the end of his time on this world. Still, he holds himself together for now, fighting with his last waking moments to see his plans through.
He gave his life to power the soulshard, allowing it to open so James could pass through. Rampart later explained more about the need for this particular action.