Shadow the Hedgehog/The Final Story

How to Unlock
To unlock the last story, you need to complete all of the endings by following good and evil paths, once you've defeated all of the final bosses such as Black Doom, The Egg Dealer, or the GUN commander and Sonic, the last story will be unlocked,if its still not unlocked, it means you missed some endings...If so, go again over the whole story and remember which paths you took and which ones you forgot to take, now after seeing what you missed or after unlocking the Last Story, select it, and the cut scenes of the real truth from shadows past shall appear one by one.

The Final Stage: "Last Way"
After the first 4 cut scenes, you go to the last stage named "The Last Way" which is the longest stage in the whole game, as the other stages, it has Secret Keys you can collect, the only difference is that in this stage there are constant Dead Ends, and you'll need to use Chaos Control to proceed, but this isn't too much of a challenge, since you can defeat lots of aliens to gain chaos control powers, once you got your gauge full, use chaos control, keep going, but remember, theres a time limit of just 10:00 to clear the stage, if you run out of time, you start again.

Neutral Mission
Find Black Doom

Dark Mission
There is no Dark Mission

Hero Mission
There is no Hero Mission

The Final Boss: Devil Doom
Despite some endings from Shadow The Hedgehog make Shadow seem an ally of Black Doom, in the end hes the real villain, to defeat it you need to calculate the right distance to shoot his eye with a Chaos Spear attack, to build this attack hold down the action button until you hear shadow saying "Chaos..." release it and he says "Spear" while shooting it, be sure not to get too close to Devil Doom, or he will just warp away from you, but you don't have to get too far from him either, or the attack just wont hit his eye, and also, remember that his eye is moving from face to face, you can know if his eye is on a face by seeing if its open or closed, if it is open, build your Chaos Spear and Shoot! continue doing this until his life gauge reaches zero. Once the battle ends you can see the real conclusion of shadows story followed by the credits, but remember, as any other sonic game you need rings in order to maintain your super form, in this boss stage you can get them by shooting your spear at the red balloons that are floating around all of the air junk, be sure your rings don't reach zero, or you'll have to restart again.