As of an unknown update, it is no longer necessary to use the Shard of Marr and follow the Spirit of Giidib to the Bells of Vhalen. You may fly or ride there and the required named mob should spawn immediately. If it doesn't then follow the steps accordingly.
Steps[]
Go to Antonica and travel to the Tower of Vhalen. ( -1300, 0, 45 ) Copy/waypoint -1300, 0, 45
Before you near the tower, activate the Shard of Marr to spawn the Spirit of Giidib. Once the Spirit of Giidib spawns, right click him, and select "Take me to the assassin."