The Hidden Legends Pokemon card expansion came out during 2004 orignally and while it might not be the oldest set of Pokemon cards out on the market the prices of this expansion has been steadly increasing in price. Currently with the value of these Pokemon cards it's still possible for collectors to build a set of them before they become as rare as the base set cards which cost a redicious price to obtain all of the cards.
These cards come in very beautiful art making them very appealing for collectors to obtain for sets. This expansion does not have first edition's and only comes in unlimited sets but it is becoming harder to find these cards in Mint condition and every collector knows that the better the condition the more valuable and rare as well as the more espensive these cards are.
Hidden Legends expansion has no known error cards that have been reported so make sure when buying cards which claim to be errored and do research to find out if they are true or just a scam by someone trying to make money.