Level up
Leveler is a mechanic introduced in Rise of the Eldrazi. Leveler cards are creatures with a keyword ability called Level up that can only be activated anytime the player could cast a sorcery. When the ability resolves a level counter is put on the creature and its level is increased to the number of level counters on the card.
When the creature reaches a certain level, it's power and toughness changes and the creature may gain additional abilities.
Creatures with leveler have a different card frame in a similar manner as Flip cards and planeswalkers were printed with a different card face. The textbox is separated into the levels with the top level showing the Level up ability with a box on the right showing the base power/toughness. Below it are boxes on the left showing the levels a change in the creature occurs with on the right being boxes with the changed power/toughness. In between those boxes is space for additional abilities if the creature gains any.
Reminder text
The latest reminder text for Leveler reads "Level up: X. (X: Put a level counter on this. Level up only as a sorcery.)"