Hollow One
Artifact Creature — Golem
This spell costs 
less to cast for each card you've cycled or discarded this turn.
Cycling
The embodiment of yearning, forever drifting the deserts to find what had once made it whole.
Anthony Palumbo
Hour of Devastation (HOU)
• #163Rulings
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.
(wotc • 2021-03-19)
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.
(wotc • 2021-03-19)
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.
(wotc • 2021-03-19)
Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.
(wotc • 2021-03-19)
Rulings
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.
(wotc • 2021-03-19)
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.
(wotc • 2021-03-19)
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.
(wotc • 2021-03-19)
Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.
(wotc • 2021-03-19)