At the end of each season all Champion League players' rating points above 5000 are transformed into the Legendary trophies. The Legendary trophies count is not reset after a season ends—it shows the activity over all league seasons. You can find them on a player's profile screen below the league icon and rating points.
NOTE: Champion league players start each new season with 5000 rating points and should complete a series of qualification battles to determine the starting league (Master/Champion).