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.