this.color.setGreen(true);
this.power = new MageInt(4);
this.toughness = new MageInt(5);
// At the beginning of your upkeep, target non-Wall creature an opponent controls gains forestwalk until your next upkeep.
GainAbilityTargetEffect effect = new GainAbilityTargetEffect(new ForestwalkAbility(), Duration.Custom,
"target non-Wall creature an opponent controls gains forestwalk until your next upkeep");
effect.setDurationToPhase(PhaseStep.UPKEEP);
Ability ability = new BeginningOfUpkeepTriggeredAbility(effect, TargetController.YOU, false);
ability.addTarget(new TargetCreaturePermanent(filter));
this.addAbility(ability);
}