Can a Champion Legend be played?

No — a Champion Legend is never "played." It's established at the start of the game: each player places their Champion Legend in the Legend Zone during setup, and it stays there for the duration of regular play (Rules 174.2.a–174.2.b, 111).

Everything it does, it does from that zone. Legends can have Passive, Triggered, and Activated abilities, can be targeted by spells and game effects, and can't be Killed or Moved (Rules 174.3–174.8). Note that non-Champion legends added by other effects are a separate case — they aren't Champion Legends and can leave the Legend Zone (Rule 107.4.d.2).