While we dont have more information about exactly what the event is or what you need to do to obtain this gorgeous card, we would guess that it would be some sort of Commander-themed event based on the promo choice
For example, the third lore counter put on a Saga causes the chapter III ability to trigger, but chapters I and II won't trigger again
Card Name Lightning, Army of One Mana Cost Type Legendary Creature Human Soldier Rarity Mythic Rare rules Text First strike, trample, lifelink Stagger - Whenever Lightning deals combat damage to a player, until your next turn, if a source would deal damage to that player or a permanent that player controls, it deals double that damage instead
To get your hands on these incredible cards, you can find Dominaria United products available for preorder at your local game store and online at Amazon.
This is Rio from the Hobby Japan FFTCG development team