The Vision is an Objective. It consists of 3 quests.
After you finished the first quest in Dumnonia, travel north from West Mercia to the settlement Eglades in Bedegraine forest. There, you will find the second quest.
First of all, you will need to choose how to get to the tower. It does not matter which of three options you will choose. When you are inside, there is Pendant of Llefelys (+4 loyalty to the hero) in the alcoves and The Truthful Sword (+3 level to Famous Leader Skill) (or some other weapon) in the unseelie room.
Hero lvl 13 is enough to accomplish this task with only 1 fight skill.
After that, you will see the third quest - The Dragon's Heart. If you are not playing Tyrant/Old Faith, you'll have to fight a powerful Unseelie army.