You engage the Oathstone, and Itharius speaks directly to your mind through the stone.
"By accepting the charge placed in this Oathstone, you agree to aid my cause and the cause of Ysera's Dragonflight in redeeming the corrupted soul of Eranikus. The first sacrifice, one of many you and I will no doubt make, will be to relinquish the gem to me that has chained what free-willed part of Eranikus' psyche remains."
"Do this <name>, and we will work together to redeem a soul worthy of redemption."
You have made your choice, and so it begins. Your choices have been, to date, what I would consider to be the correct ones; your actions and deeds henceforth will determine whether this is to continue.