Flash Reach Whenever another non-Human creature enters the battlefield under your control, you may pay {X}. When you do, put X +1/+1 counters on Wildborn Preserver.
2019-10-04
If you pay {X} while Wildborn Preserver's triggered ability is resolving, the reflexive triggered ability triggers and will resolve separately. This is different from effects that say "If you do . . ." in that players may take actions after you've paid mana but before the creature gets +1/+1 counters.