How to use

mage.abilities.effects.common.combat.CantBeBlockedByAllSourceEffect

constructor

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

public ShroudedSerpent(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId,setInfo,new CardType[]{CardType.CREATURE},"{4}{U}{U}{U}");
  this.subtype.add(SubType.SERPENT);
  this.power = new MageInt(4);
  this.toughness = new MageInt(4);
  
  // Whenever Shrouded Serpent attacks, defending player may pay {4}. If he or she doesn't, Shrouded Serpent can't be blocked this turn.
  this.addAbility(new AttacksTriggeredAbility(
      new DoUnlessTargetPlayerOrTargetsControllerPaysEffect(new CantBeBlockedByAllSourceEffect(new FilterCreaturePermanent(), Duration.EndOfCombat), new ManaCostsImpl("{4}")), 
      false, 
      "Whenever {this} attacks, defending player may pay {4}. If he or she doesn't, {this} can't be blocked this turn.", 
      SetTargetPointer.PLAYER));
}

  @Override
  public boolean apply(Game game, Ability source) {
    Player player = game.getPlayer(source.getControllerId());
    if (player != null) {
      Card card = player.getLibrary().getFromTop(game);
      if (card != null) {
        player.moveCards(card, Zone.GRAVEYARD, source, game);
        if (card.isLand()) {
          game.addEffect(new CantBeBlockedByAllSourceEffect(StaticFilters.FILTER_PERMANENT_CREATURES, Duration.EndOfTurn), source);
        }
      }
      return true;
    }
    return false;
  }
}

public RhonassStalwart(UUID ownerId, CardSetInfo setInfo) {
  super(ownerId, setInfo, new CardType[]{CardType.CREATURE}, "{1}{G}");
  this.subtype.add(SubType.HUMAN);
  this.subtype.add(SubType.WARRIOR);
  this.power = new MageInt(2);
  this.toughness = new MageInt(2);
  // You may exert Rhonas's Stalwart as it attacks. When you do, it gets +1/+1 until end of turn and can't be blocked by creatures with power 2 or less this turn.
  Effect effect = new BoostSourceEffect(1, 1, Duration.EndOfTurn);
  effect.setText("it gets +1/+1");
  BecomesExertSourceTriggeredAbility ability = new BecomesExertSourceTriggeredAbility(effect);
  effect = new CantBeBlockedByAllSourceEffect(filter, Duration.EndOfTurn);
  effect.setText("and can't be blocked by creatures with power 2 or less this turn");
  ability.addEffect(effect);
  this.addAbility(new ExertAbility(ability));
}