Under The Hunter's Moon stands as a shining example of what single-player campaign mods should be. Drawing upon the lore that players will already be familiar with, the initial narrative setup will give the, uh, victims of this mod a powerful, definitive beginning and end point. This means that the player immediately has a good concept of what is meant to be achieved, why it must be achieved and how said achievement is going to be carried out - or so they think.
Following the plot of the first Aliens vs. Predator film, the mod sees you, a Predator, dropped at an Antarctic mining station on planet Earth in order to complete your blooding ritual. You goal is to engage in combat with the Aliens and make it out of there in one piece. You're set down with two buddies a small distant away from a precision, laser-drilled hole in the ice that will take you down to the cross-culture pyramid. Your two friends are basically cruise-control for survival. They'll fight alongside you or stay put depending upon your orders. In every level, they are set to twiddle their thumbs and jerk off while you aren't looking by default, so you'll have to go tell them to get their shit together first thing.
Finding the entry point would be a pretty mundane task if there wasn't at least a dozen or more armed humans in the way. This brings me to my first real criticism - there's absolutely minimal human combat in the game. After this section, you'll rarely meet or fight human prey again, and usually they'll be alone. What tempers this criticism is how difficult it would make the mod. There's no energy sift, so when energy is used, it is used. Until the next loading screen, that is. The point is that your energy is quite severely limited, so when you meet a human enemy you seldom have the energy to spare to cloak properly and set up an easy kill while they are still very capable of chewing you up with their firearms.
Human combat is only so difficult as the mod goes on because the Aliens have drained all your resources. They're absolutely fierce and deadly. Forget taking them on in melee for fun, you're going to have to get up close and you'll hate it. Depending on how well you're defending yourself, Aliens can kill you with a single strike, whereas it often takes the combistick two strikes to dispatch one. The game is scripted so that they're just downright better in hand-to-hand combat than you are, and your ranged weapons are limited enough that you'll often be forced into the melee. Every engagement with the Aliens, no matter how small, is a challenge and a risk.
There are a number of ways to increase your survivability with the items you pick up. One of the items in your inventory is a javelin. This is a one-use, one-hit-kill weapon that is thrown a fairly short distance with a gravity arc. If you find the enemy you threw this at, you can recover it, although you may not need to as there's dozens of these scattered around the pyramid that you can pick up. The second on the list of survival tools is the netgun, which actually has fairly plentiful ammo. As long as you aim your shots properly and use your ammo with efficiency, you'll usually have at least a few nets at your disposal and they're incredibly useful for taking Aliens out of the battle temporarily to ease the strain on combat or preventing them from fighting back while you kill the shit out of them.
The other two weapons, the disc (or "shuriken" in this case) and plasma caster, rely on energy. Since there's no medicomp and cloaking is useless against Aliens, your energy bar is essentially just an ammo counter for using these weapons and as soon as you find the plasma caster, the disc is fairly useless. It simply requires too much energy for too little damage, where the plasma caster gives you a respectable number of shots with which to destroy Aliens. Overusing the plasma caster is easy and will leave you out of energy for those moments when you really, really just need to blast about a dozen Aliens at once, so it's generally better to use the netgun and finish off Aliens with a javelin throw or combistick attack (or two).
While the game is in FPS format, the combat causes a kind of survival horror atmosphere and there's enough puzzle elements to have you scratching your head over non-combat sections as well. There are plenty of traps to force you to more carefully consider your progression and the way forward is rarely exactly clear. It's not surprising at all that plenty of players don't finish this without cheats, since it's quite a bit more than most FPS fans would expect.
Challenging and fun, with plenty of tension and level design that fits both the theme and gameplay extremely well. If I assigned numbers to mods, this would get a fairly high one.