I sent this very brief summary to Sven - might help answer a few questions:
The Glintshield clan used to have a stronghold and run the pass - the only way across the Ironwall mountains.
Then 100 years ago they mysteriously disappeared. Rumours of infighting and civil war.
Recently the Glintshield clan reappeared and started rebuilding the citadel and re-opening the pass.
Then their blood enemies - the Severed Eye orcs, attacked them. Those dwarves not killed, fled into the pass and sealed it behind them.
The Shadowed Chain were a secret cult inside of the Glintshield clan.
The civil war was over the theft of the rib - used to bind the titan.
The cultists had managed to recover the rib and spent decades convincing the Thane to reopen the pass. They hoped to return the item to the binding site when the pass was made safe, but fate - and the orcs - intervened.
This is why they had the rib. The other parts were stolen from the binding site and were in possession of the orcs.
Note that it is not possible to bind Stonefang without all 4 parts, but one part can be missing for some time (as evidenced by the stolen rib).