Thornwood is in between intermediate and end game difficulty grind zone wise. Thornwood mobs are primarily grinded for money and the costume piece. The La Orzeca Outfit is a new outfit that can only be obtained by grinding and obtaining a lung. Once the lung has been gained you can turn it into an NPC in exchange for one of the costume pieces. Thornwood drops the Left lung. If you cannot obtain a lung drop you can also get black leaf's. Once 100 leaf's have been obtained you can use them on obtaining a costume piece as well.
Thornwood is located east of Grana; Grana is located South of Calpheon, the third major city in Black Desert.