Arna's attack trigger essentially enables attacking Living Weapon creatures to split off and make their controller's board wider, or go-tall with stackable effects like Drossclaw's draining attack trigger or Nettlecyst's massive boost based on artifacts and enchantments
While card draw is a huge way to maintain control in a game, forcing your opponent to discard helpful Items, Songs, and Actions can knock the wind out of them and deflate their strategy before it even gets up and running
She is very much about the -3 ability, killing a key threat and leaving you with both a planeswalker and a Treasure (which is especially nice with Fatal Push)
Once per turn, the user would be able to summon a level 4 or lower monster from the deck to the field , at the cost of not being able to invoke it from the hand
Legendary Creature Human Warlock When Ultimecia enters, tap all creatures your opponents control