In ESO players can transform into monsters to help give you distinct advantages in battle.
Monster Advantage System
Like pets, each transformation type is part of a monster family. Transforming into a specific monster type will give you distinct advantages over other monster families. To transform into a monster, you would have to obtain a transformation card. There are multiple ways to get new transformation cards.
Each time you kill a monster there is a chance a transformation card may drop.
You can purchase it from other players.
Certain quests may reward you with transformation cards.
Earn monster bond by doing bulletin board quests and trade in your bonds in Pokari for transformation cards.
Each card you obtain will usually last for 50 uses.
Below is an example of the transformation menu.
Transformations are most useful in the monster advantage system. Notice a little icon near the monster name when a monster is selected? That is the monster's family. There are nine families in the game, each with strengths and weaknesses against another family. Below is a chart of the each family with arrows designating its advantage and disadvantage.