// Whenever Rhys the Exiled attacks, you gain 1 life for each Elf you control.
this.addAbility(new AttacksTriggeredAbility(new GainLifeEffect(new PermanentsOnBattlefieldCount(filter2, 1)), false));
// {B}, Sacrifice an Elf: Regenerate Rhys the Exiled.
Ability ability = new SimpleActivatedAbility(Zone.BATTLEFIELD, new RegenerateSourceEffect(), new ColoredManaCost(ColoredManaSymbol.B));
ability.addCost(new SacrificeTargetCost(new TargetControlledCreaturePermanent(1, 1, filter, false)));
this.addAbility(ability);
}