Walking Ballista enters the battlefield with X +1/+1 counters on it.
: Put a +1/+1 counter on Walking Ballista.
Remove a +1/+1 counter from Walking Ballista: It deals 1 damage to target creature or player.
2/9/2017 The value of each X in Walking Ballista’s mana cost must be equal. For example, if X is 2, you’ll pay to cast Walking Ballista and it will enter the battlefield with two +1/+1 counters on it.
2/9/2017 If Walking Ballista has been dealt damage or had its toughness reduced by an effect, this limits how many times you’ll be able to remove +1/+1 counters from it in a single turn. For example, if it has three +1/+1 counters on it and has been dealt 1 damage this turn, it will be destroyed immediately after you activate the ability a second time and you won’t be able to activate it a third time.