To get this trophy you must complete the quest "The Fallen" which is a part of the main story and cannot be missed.
*** Spoiler - click to reveal *** After defeating Alduin in the quest "Alduin's Bane" Paarthurnax will suggest you try to trap one of is allies to find out where he has gone and that you may be able to trap one of them in Dragonsreach. Go speak with the Jarl of Whiterun and convince him to help you with trapping a dragon. Before he goes through with the plan, he will ask you to talk to the Graybeards about hosting a peace summit between the Stomrcloak and the Imperials.
Travel to High Hrothgar and speak with Arngeir. After he agrees to host the peace council you will begin the quest "Season Unending" which tasks you with speaking with both General Tullius of the Imperials in Solitude and Ulfric Stormcloak in Windhelm. If you have not yet discovered these locations you can take the carriage to either of them. After convincing them to attend the peace council, return to High Hrothgar and speak with Arngeir again and begin the peace council.
During the negotiations you will be asked to agree or disagree with what has been proposed by both sides. Ultimately both the Imperials and the Stormcloaks will come to an agreement. Go speak to Esbern who will teach you the call dragon shout. Return to Dragonsreach and speak with Jarl Balgruuf. Go outside to the balcony and use the call dragon shout to summon Odahviing. Once he shows up use Dragonrend to get him to land so the guards can trap him.
After he is trapped, interrogate him to find Alduin's location. After you finish speaking with Odahviing, the quest will complete and the trophy will pop.
*Cross-posted from Skyrim (PS3) trophy guide