Hexen/Sacred Grove

In Shadow Wood, climb up the rocky pinnacle near where you began the episode. The small cavern near the top opened after you pulled the six puzzle-piece levers. Inside is the portal to Sacred Grove.

Sacred Grove
This is the smallest and simplest level in the game, as well as one of the nicest, monsters aside: a large, mostly-square, foggy area with a column in the middle and a huge quantity of mana and items everywhere. First, quickly eliminate all of the Ettins wandering around. If you take too long, additional Ettins will begin spawning in groups of four at regular intervals, and things can quickly get out of hand. You'll get "x Ettins left" messages when this occurs.

When they're all taken care of, wait a few seconds. The ground surrounding the column will lower in concentric rings, forming something like a 360-degree amphitheater of sorts. Look at the part of the column that's below ground level, and pull the lever you find. The "Forsaken Outpost" the message mentions is a level in the fourth episode, and the lever has allowed access to that episode's secret level. This is why coming to this level is critical.

Bonuses
Look carefully along the vines surrounding the area. At the centermost point on the three sides other than the one with the exit portal, you'll see small areas hidden behind the vines. These are flagged 'secret" and won't show on the automap. In all three areas you'll find Mystic Urns.