Hero of Leina Tower
Creature — Human Warrior
Heroic — Whenever you cast a spell that targets this creature, you may pay 
. If you do, put X +1/+1 counters on this creature.
"You cannot achieve greatness if its seed has not already been planted in your heart."
Aaron Miller
Born of the Gods (BNG)
• #123Rulings
The amount to pay for {X} is chosen as the triggered ability resolves. No player may take actions between the time you pay {X} and the time Hero of Leina Tower has X +1/+1 counters on it.
(wotc • 2018-12-07)
Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.
(wotc • 2013-09-15)
Heroic abilities will resolve before the spell that caused them to trigger.
(wotc • 2013-09-15)
Heroic abilities won’t trigger when a copy of a spell is created on the stack or when a spell’s targets are changed to include a creature with a heroic ability.
(wotc • 2013-09-15)
Rulings
The amount to pay for {X} is chosen as the triggered ability resolves. No player may take actions between the time you pay {X} and the time Hero of Leina Tower has X +1/+1 counters on it.
(wotc • 2018-12-07)
Heroic abilities will trigger only once per spell, even if that spell targets the creature with the heroic ability multiple times.
(wotc • 2013-09-15)
Heroic abilities will resolve before the spell that caused them to trigger.
(wotc • 2013-09-15)
Heroic abilities won’t trigger when a copy of a spell is created on the stack or when a spell’s targets are changed to include a creature with a heroic ability.
(wotc • 2013-09-15)