Enter Antonica and travel to the Tower of Vhalen. ( -1300, 0, 45 ) /waypoint -1300, 0, 45
Before you near the tower, activate the Shard of Marr to spawn the Spirit of Giidib. (The spirit walks at a slow pace. Activating the shard nearer to the tower will allow you to travel faster.) Once the Spirit of Giidib spawns, right click him, and select "Take me to the assassin."
[25/10/2015 on Splitpaw (EU)]: Approached Tower of Vhalen on flying mount. Without interacting with the shard at all, Viragos, the Assasin of the Hand spawned (level 29^). Upon slaying him the quest skipped to step 5. above. It appears the shard has been rendered superfluous.