Road to Gehenna builds on the The Talos Principle, a philosophical first-person puzzler from Croteam. It takes the best elements of Portal, but removes the light-hearted humour and replaces it with deep levels of intense, existential dread, along with one of the most beautiful atmospheres I've experienced in gaming.
If you haven't played The Talos Principle yet, go and do so now. Partly because I don't want to spoil the storyline when I talk about Gehenna, but mostly because it's shitting brilliant.
The game takes place after your ascension in the previous game. You now play a messenger of Elohim, travelling through new areas in the same world to rescue others that have failed to ascend. Elohim had trapped them, but now believes he was mistaken and asks you to set them free, to ascend the restraints of this dying world. These others now face the same dilemmas you faced as the protagonist in the first game: they ask whether the world they are to be rescued into is better than their current prison, and question you to that effect. How can they even trust you? How do you even know you're doing the right thing? Oh, and of course, how do you know you even exist?
In between solving the perfectly crafted physics puzzles, you will talk to the AI characters via a forum, accessed via computer terminals that are scattered across the world. Sometimes to speak about the happenings of the world and the job you must do for Elohim, and sometimes to talk about the creations of the artificially intelligent beings - they often require you to critique their artwork...
It's not clear how long the various AIs have been trapped in this world, but they've built an almost endearing little community, and have begun to develop their own art and culture. There's even a playable text-based RPG, and a digitised art gallery. A standout piece of art for me was entitled "Existence", consisting of a single zero in the centre of an otherwise empty screen, mainly because of how it contrasted with the nearby work of another AI: an ascii-art picture of a sad giraffe.
You will essentially make your way around this beautifully crafted world, freeing AIs one puzzle at a time. However, this expansion provides way more than extra puzzles to solve, it manages to continue the air of mystery and incredible atmosphere the initial set of levels oozed. These new levels are also much bigger than those found in the first game, and less separated from one another. Instead of feeling restrained by the boundaries of the level, you now only feel restrained by the boundaries of the world that the puzzles exist within. In fact, not all puzzles exist in their own chambers...
The puzzles are different enough to feel exciting, difficult enough to feel rewarding, and infuriating enough to make you want to hurt yourself. There are several optional goals (stars to collect) that are much more challenging than any found in the first game. One such star took me around 4 hours to get, and involved such a complicated and carefully timed chain of events, I'm still in utter disbelief that the whole thing came together for me. There's usually an element of "What? That actually worked?!" whenever you complete one of the more difficult levels.
In summary, Gehenna is an utterly essential expansion. If you liked the first game (or Portal) even a little bit, you will love this.
For myself, a full sequel to Talos is as eagerly awaited as Half-Life 3 remains.
Oh, and there's also a plastic leprechaun.