After completing City of Thieves in the Barahut portal, there will be a scene where Mary is kidnapped. This will open the Forest, which is accessible from the room in which Mary was kidnapped.
In the Forest, you'll be chased by the monster seen during the kidnapping and will have to fight through two mini boss sections. During the chase your path will be blocked by corruption that appears to only subside when the monster is close.
Eventually you'll fight the monster. During this fight you can collect gemstones to activate the small obelisks similar to the mini boss sections. These are limited to about 3 per obelisk. Each activation damages the bosses, kills minions, clears corruption and heals you. The boss is rather easy using Linda, as she can fire at the boss from behind an obelisk. Clear the adds and activate an obelisk as needed.