When Lethrai Skystrider attacks, your attacking Elves get Flying this turn.
Elf Ally: Draw a spell from the top 4 cards of your deck. Discard the rest.
Card sounds may differ from in-game as they get updated.
Flying - Flying units can only be blocked by other flying units, but can block ground units normally. They can also be directly attacked by Killer units and Relic Weapons.
Ally - Happens as soon as you have a unit of the named type. This can be when you play the unit with Ally or later if there's none of the needed type in play. Ally resets if the unit dies or is returned to hand.