If the land this Aura would enchant is an illegal target by the time New Horizons resolves, the entire spell doesn't resolve. It won't enter the battlefield, so its ability won't trigger.
03-05-2019
wotc
You can cast New Horizons even if you control no creatures.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.
: Add two mana of any one color.”
Noah Bradley2020-07-17
