I am at the part where Bugenhagen tells me to go to a place where sunlight..

To get there you need the submerine because it's in a cave under water (watch out for emerald weapon swimming around)

I'm sorry I couldn't tell you exactly where it is. I forgot.

It's under the sea near the north go there with the sub.

Ok you need the sub for this.its around the southern area of the northern continent.

Check the sea area west of bone village.

Then you can go underwater(the emareld weapon does not swim around this area so don't worry if you fear him)and you enter a cave.

The key is in there.

You can only get to this key by going through the sealed doorway, which you couldn't go through earlier.

Bugenhagen will stand in front of it when you get there, and open it. You go through what is called the "Cave of the Gi" after the doorway.

Near the end of the cave you will fight a boss, like Gi Chieftan or something, he is a Zombie so he takes damage from any form of healing.

After the fight Red 13 will see a large petrified form of his father.

After a long cut scene, the statue will cry and you will recieve Red 13's ultimate weapon.

Well first you must have the submarine.

Go under the sea and swim all the north-east you can go. there should be a underwater cave.

Go in it and the key to the anceints will be there.

Watch out about swimming underwater, emrald waepon will be swimming around (the gaint green monster).

He will be way,way,way,way too strong for you now.