Legend teams are special teams that appear during a Legend Challenge.
Each Legend team features different players, formations and rewards.
There are also Rare Legend Teams!
You will see a time limit at the bottom left of the Legend Challenge screen indicating the time left to play this particular Legend Challenge match.
If this time limit reaches zero before you choose to play or quit a match, the Legend team will leave and you won't be able to play a Legend Challenge until the next time they appear.
If you decide to challenge them, be sure to tap 'PLAY!' before the timer runs out.