Underdark Beholder — ruling 1

Underdark Beholder’s second ability isn’t a damage prevention effect. It can replace damage that can’t be prevented.

Underdark Beholder — ruling 2

If Underdark Beholder leaves the battlefield while its last ability is on the stack, use the number of eyestalk counters it had before it left to determine what converted mana cost to look for.

Underdark Beholder — ruling 3

You must cast the revealed card while Underdark Beholder’s triggered ability is resolving. You can’t cast it later in the turn.

Underdark Beholder — ruling 4

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Underdark Beholder — ruling 5

If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, you must pay those to cast the card.

Underdark Beholder : règles, fonctionnement et interactions (MTG)

Underdark Beholder 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

Underdark Beholder enters the battlefield with ten eyestalk counters on it. If Underdark Beholder would be dealt damage, remove that many eyestalk counters from it instead. If you can't, sacrifice it. Whenever Underdark Beholder attacks, reveal cards from the top of your library until you reveal an instant, sorcery, or enchantment card with converted mana cost less than the number of eyestalk counters on Underdark Beholder. You may cast it without paying its mana cost. Shuffle your library.

How it works : Underdark Beholder

Original English text — Wizards of the Coast / Scryfall

#1November 12, 2019· Wizards of the Coast
Underdark Beholder’s second ability isn’t a damage prevention effect. It can replace damage that can’t be prevented.
No other card is named in this ruling.
#2November 12, 2019· Wizards of the Coast
If Underdark Beholder leaves the battlefield while its last ability is on the stack, use the number of eyestalk counters it had before it left to determine what converted mana cost to look for.
No other card is named in this ruling.
#3November 12, 2019· Wizards of the Coast
You must cast the revealed card while Underdark Beholder’s triggered ability is resolving. You can’t cast it later in the turn.
No other card is named in this ruling.
#4November 12, 2019· Wizards of the Coast
If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.
#5November 12, 2019· Wizards of the Coast
If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, you must pay those to cast the card.
No other card is named in this ruling.