Creatures in MagicDuel are under your command, and battle for you against other players.
You start out with 16 creature slots, but can purchase more via the MDShop if you so desire. One creature fills one creature slot.
Each creature (or critter, as they are informally known) has different abilities and characteristics. Most creatures can be upgraded - once certain requirements are met - into new and more powerful creatures. The requirements for upgrading creatures can include: Age, VE, VP, Heat & Creature Win Count (Wins).
Creatures can be sacrificed at various altars throughout the MagicDuel Realm to gain special rewards. The more powerful a creature is when sacrificed, the greater the reward. Different creatures mean different rewards.
You can check on your creatures at any time by clicking on the Creature icon within the Player Toolbar (seen selected in the image to the left)
Within this page, you can upgrade creatures (if the requirements mentioned above are met), trade creatures to other characters or restore a creatures VE by clicking on ther individual VE bars. Note that every click requires 1 Action Point and that only a certain amount of VE will be given to a creature with each click, depending on how much VE you actually have. In order to increase the amount of VE given per click, consider acquiring some adepts.