Champions are one of the unit cards that a player can own in the game Legends of Runeterra. However, unlike Followers, they are much more powerful and have complex mechanics. Because of that, decks often build around champions.
A player can own duplicates of a champion, however, when the champion is active, their duplicate cards will transform into special spells, similar to the ones existing, with a unique addition action: Shuffle a copy of the champion card into the player deck.
For example: Player is owning 3 Ezreal cards. When an Ezreal is played, the other 2 cards will transform into Ezreal's Mystic Shot. In addition to the normal Mystic Shot effect, Ezreal's Mystic Shot shuffles an Ezreal card into the player deck.
However, this doesn't mean that multiple champions can't be on the board, as there are various effects that can in some way allow you to have a duplicate, Dawn and Dusk, Splinter Soul, and The Rekindler to name a few.
Level upLevel Up is a keyword that causes a champion card to transform into another specified card, as well as transforming all allied ones of the same card everywhere, living or dead.
Each Level Up has a corresponding trigger condition and the transformation occurs at Burst speed immediately following the condition being met. For example,
Level Up: I have attacked twice. would cause the card to transform after it performs its second attack.
The transformation persists for the remainder of the game even if the champion is returned to the hand or dies. All Champions possess this keyword, and some may Level Up multiple times.
In normal game mode, a player can have at most 6 champions in their decks. These cards can be from different champions, or have duplicates. Just like other cards, a player can have at most 3 duplicates of one champion.
In Expedition, a player has 2 champions in their initial deck. As the player progress in a trial, they can earn more champions cards.