How to be Batman

You may have noticed players in Cyrodiil that are crushing large groups of players single-handedly through use of the Vampire ability Devouring Swarm (bat swarm AoE). You may also have seen videos that show how strong players can become with clever use of this ability. You may not know how this is happening. Here's my explanation:

When your vampirism reaches stage 4, your activated vampire skills' cost is reduced by 60%. This includes the ultimate. There are methods of further reducing the cost.

The Akiviri Dragonguard Set will reduce the costs of ultimate abilities by 20%. This set drops in Glenumbra at veteran level.
The Seducer set will reduce the costs of ultimate abilities by another 3%. This set can be crafted by players.