Benusta:
Spirit Forest is a place near Venom that is crawling with monsters. I fear for the safety of the villagers. I've also heard that they can become invisible. You must eradicate them.
The requirements to enter the Spirit Forest are not different from other instance zones. Although you should stay inside the zone even if you die. And if the whole party leaves the Spirit Forest, a request to re-enter the zone should be made within 1 minute.
Also you need to select the difficulty after entering the zone. Regardless of the selected difficulty and your results you cannot re-enter the Spirit Forest instance zone until the access limit resets at 06:30.