Abilities are unlike previous assassins creed games because you have to find them. They appear first as gold blobs which turn into gold books of knowledge when located. The first one you can spot is in Nottfall (I wasn’t able to get this though).
These books contain varying abilities which can equipped.
You can have 4 Ranged (left) and 4 Melee (right). When you have 8 equipped the trophy is yours.
From each book you learn 1 ability but find another of the same and you can use grade it.
You can also gain abilities as quest rewards.
1 ability for example is
Mark of Death — Hold to mark sighted enemies. Release to let fly a deadly volley of arrows