How to use

ChooseSecretOpponentEffect

in

mage.abilities.effects.common

@Override
public ChooseSecretOpponentEffect copy() {
  return new ChooseSecretOpponentEffect(this);
}

public EmissaryOfGrudges(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId, setInfo, new CardType[]{CardType.CREATURE}, "{5}{R}");
  this.subtype.add(SubType.EFREET);
  this.power = new MageInt(6);
  this.toughness = new MageInt(5);
  // flying
  this.addAbility(FlyingAbility.getInstance());
  // haste
  this.addAbility(HasteAbility.getInstance());
  // As Emissary of Grudges enters the battlefield, secretly choose an opponent.
  this.addAbility(new SimpleStaticAbility(Zone.BATTLEFIELD, new EntersBattlefieldEffect(new ChooseSecretOpponentEffect(),"As {this} enters the battlefield, secretly choose an opponent.")));
  // Choose new targets for target spell or ability if it’s controlled by the chosen player and if it targets you
  // or a permanent you control. Activate this ability only once.
  Ability ability = new SimpleActivatedAbility(Zone.BATTLEFIELD, new EmissaryOfGrudgesEffect(), new RevealSecretOpponentCost());
  ability.addTarget(new TargetStackObject());
  this.addAbility(ability);
}

public StalkingLeonin(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId, setInfo, new CardType[]{CardType.CREATURE}, "{2}{W}");
  this.subtype.add(SubType.CAT, SubType.ARCHER);
  this.power = new MageInt(3);
  this.toughness = new MageInt(3);
  // When Stalking Leonin enters the battlefield, secretly choose an opponent.
  this.addAbility(new EntersBattlefieldTriggeredAbility(new ChooseSecretOpponentEffect(), false));
  // Reveal the player you chose: Exile target creature that's attacking you if it's controlled by the chosen player. Activate this ability only once.
  Ability ability = new SimpleActivatedAbility(Zone.BATTLEFIELD, new StalkingLeoninEffect(), new RevealSecretOpponentCost());
  ability.addTarget(new TargetCreaturePermanent(new FilterCreatureAttackingYou()));
  this.addAbility(ability);
}