Maro's Gone Nuts

Enchantment

Double any effect that doubles. (It quadruples.)

2019-11-12:
An effect “doubles” if it instructs a player to double something, or if it replaces an event with “double that much,” “twice as many,” or similar.
2019-11-12:
Double strike isn’t an effect, so it’s not doubled.
2019-11-12:
An effect that increases something by a specific amount (such as the first ability of Jaya, Venerated Firemage) isn’t doubling, even if the result is double the normal result.
2019-11-12:
If two Maro’s Gone Nuts are on the battlefield, effects that double instead octuple. If three are on the battlefield, effects that double instead sexdecuple, and so on.