Once The Ancient One has legally attacked or blocked, removing permanent cards from your graveyard so the descend 8 ability no longer applies won't remove The Ancient One from combat.
The Ancient One
Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.
Cards with the ability word "descend N" have abilities that care if you have at least N permanent cards in your graveyard.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Victor Adame Minguez
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