Mixing eerie, survival horror adventure with puzzle-heavy first-person exploration, the Penumbra series straddles the line between distinctly different genres - much like its unfortunate protagonist Philip's mind roams in and out of the depths of insanity. Originally intended as a trilogy, the series was inexplicably squeezed into two installments. Fans bemoaning the brevity of the second excellent chapter, Black Plague, released earlier this year, will derive some additional enjoyment from the puzzles found in Penumbra: Requiem.

Though Black Plague was indeed the final full chapter in the short Penumbra series, Requiem serves as a brief epilogue to tie up loose ends and stretch the engaging gameplay a little further. It surprisingly ditches much of the straight-up horror elements of the first two games and places a greater focus on exploration and puzzle-centric problem solving. The creepy vibe and dimly lit environments are still there, but it's a lot less nerve-wracking knowing you're not going to get mauled by a slathering infected beast while trying to figure out how to escape each level. Giving more weight to the puzzle aspect gives Requiem a decidedly different feel to it. Taking some of the edginess away doesn't hurt the gameplay at all, though it's a move that may not appeal to all players who enjoyed the rest of the series.

After stripping out what little combat and enemy confrontations were left in the previous game altogether, the action begins to feel very similar in structure to Portal. Every area is completely self-contained, and each presents a different environment and set of challenges to overcome. The main objective is to solve a series of puzzles using found objects and intuition in order to locate all the hidden keys to power up (ironically) an escape portal. The similarities, and there are others, to the aforementioned Valve title are quite noticeable, but this final, short installment is clearly more of a fun add-on to wrap-up the story and extend the playability of the Penumbra series than a direct Portal rip-off.

The nine levels in Requiem, each of reasonable length, are wrapped together by a thin continuation of where the story left off in the last installment. However, the game is light on plot, and the meager story elements present can be hard to follow at times. Rather than being propelled by action-driven events, it progresses on a more psychological level. Without giving anything of major importance away, it's safe to say it does provide a clever (though slightly anticlimactic) conclusion to the series.

Melding first person shooter, point-and-click adventure, puzzle, and survival horror genres into one title is a bold move, but Penumbra pulls it off without a hitch. Like the previous games, Requiem features a simple first-person shooter controls scheme that utilizes the WASD keys to move and the mouse to turn and look around. Instead of firing a gun or swinging a weapon - ranged and melee elements have been completely removed in this installment - the mouse buttons allow you to pick up and interact with objects. Aside from simply grabbing and dropping (or throwing) items, some things like doors, drawers, cranks, and levers require you to click on the object and then make an appropriate motion with the mouse to engage the action. Most objects you pick up like barrels, garbage, and metal pipes remain in your grasp and move with you until you let them go, use them on the appropriate hotspot (in some cases), or check them into the darkness. This completely negates the need to dig around in your inventory - something that's essentially useless throughout this expansion pack.

Requiem's puzzles are not your typical adventure game fare. Though there is an inventory system and a very limited array of items - primarily a flashlight to illuminate the dark environs and painkillers to restore your health - there's virtually no item management or combining needed. Instead, you primarily manipulate 3D objects and use them in the game environments in real-time. You'll be doing a lot of switch-hitting, stacking barrels to reach higher areas or weight down pressure sensitive triggers, pulling levers, inserting objects, and so forth.