In a disappointing move, the international release of 151 does not include the Japanese versions wonderful reverse-holo pok ball pattern, which came with a chance that they could instead feature master balls, and as such become much more valuable
Certification: New Cert
It's also one of the only ways to put additional counters on Breaker the Magical Warrior for easy spell and trap destruction
Cavern Harpy is the card that made the Aluren decks run by letting you duplicate effects on queue, with the rest of the pieces being able to fluctuate with the format and times
[1] Special Summon 1 Level 7 or lower "Archfiend" monster from your hand or Deck, if a face-up "Archfiend" Ritual monster is in your Extra Deck, you can Special Summon up to 1 of each "Royal Archfiend", "Duke Archfiend" and "Highness Archfiend" from your hand or Deck instead
In this case, it searches one out from the Deck or Graveyard, then can put it on the board immediately