Follow Talon to this wall. It dissolves as you approach, allowing passage. This outdoor environment isn't exactly wide open, but it is a welcome sight after crawling corridors. Talon is on the ledge ahead for the moment. When you move forward, he'll fly to the right.
Follow Talon through the passage to the clearing ahead.
Enisi appears, explaining that you are in the land of the ancients.
Follow Talon to this door. It rolls aside as you approach.
Climb the stairs and enjoy the rare view. Outside scenes like this are uncommon in Doom 3-engine games.
Step into the cavern and approach this symbol. Once again, the door dissolves.
Do as Enisi says and learn to astral project, or "Spirit Walk."
Pass through the flames then press the fire totem to extinguish the blaze.
Return to your body as directed.
You can now proceed to the spirit cave ahead.
Your new weapon, the spirit bow, appears amidst rings of fire.
Spirit Walk through the flames to claim your bow. Enisi explains that the bow must be replenished with the spirits of your fallen enemies (sort of like Doom 3's Soul Cube).
Enisi sends you back to the physical realm, explaining that Talon will be your eyes and ears upon your return.