The Underground Cemetery is just below the Underground Reservoir. When you first enter, you will only encounter very few monsters, and hear an eerie moan. The only thing you can reach is the save point just before the boss door.
When you go through, you will encounter Legion. It is a large cluster of dead bodies fused into a huge ball. Its main attack is to approach you slowly and try hitting you with collision damage. However, it will also release bodies that will slowly walk around on the ground or platform they land on. Both the core and segments have separate hit point pools, and the enemy is defeated once you take out the core.
When you destroy one segment, Legion will begin to shoot a laser. If the lower two are destroyed, you are able to freely slide under it and to press the attack. When all four, it will be able to fire three lasers, and occasionally make a sudden bounce around the room.
To collect the legion soul, you must destroy all four sections before taking out the core. You will then be able to collect the Galamoth soul by continuing to the right, which allows you to get past the Chronomage in Inner Quarters.
There are two items of note - the first is the Gold Ring at the top-right corner of the cemetery, and the other is $2000 at the top-left corner.