Hollow One — ruling 1

Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.

Hollow One — ruling 2

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.

Hollow One — ruling 3

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.

Hollow One — ruling 4

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.

Hollow One : règles, fonctionnement et interactions (MTG)

Hollow One is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast for each card you've cycled or discarded this turn. Cycling (, Discard this card: Draw a card.)

How it works : Hollow One

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#3March 19, 2021· Wizards of the Coast
Once you've discarded three cards during one turn, Hollow One costs to cast. It won't stop at or cost negative amounts of mana.
No other card is named in this ruling.
#4March 19, 2021· Wizards of the Coast
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.
No other card is named in this ruling.