Champion allows you to upgrade one permanent into another, representing a powerful creature taking the place of another to fight in its stead. It was introduced in Lorwyn, in which it features on a cycle of rare non-shapeshifter creatures and on three uncommon shapeshifters with changeling, in whose case "champion" appears as "champion a creature".   Morningtide featured cards that let you champion one of two creature types, the player gets to choose which one. 
From the Comprehensive Rules (Born of the Gods):
- 702.71. Champion
- 702.71a Champion represents two triggered abilities. "Champion an [object]" means "When this permanent enters the battlefield, sacrifice it unless you exile another [object] you control" and "When this permanent leaves the battlefield, return the exiled card to the battlefield under its owner’s control."
- 702.71b The two abilities represented by champion are linked. See rule 607, "Linked Abilities."
- 702.71c A permanent is "championed" by another permanent if the latter exiles the former as the direct result of a champion ability.
- A creature's own champion ability won't allow you to remove that creature from the game.
- If a creature with champion leaves play before its comes-into-play ability resolves, its leaves-play ability will do nothing. Then its comes-into-play ability will resolve. Its controller may remove a permanent of the appropriate quality he or she controls from the game. If the player does, that card won't come back. If the player doesn't, nothing happens (because the creature with champion isn't around to be sacrificed.)
- If a creature with champion loses the champion ability (due to Humble, for example) and then leaves play, champion's leaves-play ability won't trigger. The removed card remains removed from the game.
- All Lorwyn cards with both changeling and champion have "champion a creature." The ability allows you to remove any other creature you control from the game.
- All other Lorwyn cards with champion have "champion a [creature type]." The ability doesn't limit you to creatures. For example, a card with "champion a Kithkin" allows you to remove a Kithkin enchantment from the game.
- Changeling Berserker -
Creature — Shapeshifter
Changeling (This card is every creature type at all times.)
Champion a creature (When this enters the battlefield, sacrifice it unless you exile another creature you control. When this leaves the battlefield, that card returns to the battlefield.)
- Thoughtweft Trio -
Creature — Kithkin Soldier
First strike, vigilance
Champion a Kithkin (When this enters the battlefield, sacrifice it unless you exile another Kithkin you control. When this leaves the battlefield, that card returns to the battlefield.)
Thoughtweft Trio can block any number of creatures.
There is a total of 12 cards that involve champion, which divide by color as such:
- Blue = 25 %
- Green = 25 %
- Red = 25 %
- White = 17 %
- Black = 8 %
- ↑ English Rules Primer — Lorwyn
- ↑ Mark Rosewater. (October 1, 2007.) "And the Rest", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Mark Rosewater. (October 08, 2007.) "Before and After", Daily MTG, magicthegathering.com, Wizards of the Coast.
- ↑ Mark Rosewater. (January 07, 2008.) "But Wait, There's More", Daily MTG, magicthegathering.com, Wizards of the Coast.