Once you've discarded three cards during one turn, Hollow One costs {0} to cast. It won't stop at {1} or cost negative amounts of mana.
19-03-2021
wotc
Hollow One's first ability doesn't give you permission to discard cards. You'll need another effect that instructs or allows you to discard them, such as a cycling ability.
19-03-2021
wotc
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Hollow One). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.
19-03-2021
wotc
Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.
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.