Eco didáctico (Didact Echo)
Eco didáctico 

Descender 4 — El Eco didáctico tiene la habilidad de volar mientras haya cuatro o más cartas de permanente en tu cementerio.
Texto de reglas (Oracle, inglés)
Descend 4 — This creature has flying as long as there are four or more permanent cards in your graveyard.
Los habitantes del Imperio del Sol respetan a sus ancestros. Los oltec van más allá y se comunican con los espíritus de sus mayores más queridos después de la muerte.
Legalidad
Aclaraciones de reglas (en inglés)
2023-11-10
Cards with the ability word "descend N" have abilities that care if you have at least N permanent cards in your graveyard.
2023-11-10
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.