One of the most impressive achievements of the mod. This revolves around a major character dying, hence the spoiler section. Unlocking this achievement will also unlock:
*** Spoiler - click to reveal ***Paul needs to die. Personally, I died during the attack in the apartment in NYC. You can find Paul's body in the medical area of the MJ12 facility, after you escape from your cell. Here's the tough part. You need to physically carry his corpse for several chapters, until you meet Dowd in his family mausoleum. As I progressed the story I always made sure the coast was clear, since you move more slowly and can't shoot when carrying a corpse, but you can swim, open unlocked doors and climb ladders. Be mindful of explosions when you've put the body down temporarily, as it can be destroyed.
Some say you can leave it at certain places where it's safe and where you know you'll return, for example the top of the building right before the big fan in the PRCS Wall Cloud level. I was too scared of it despawning though, so I took Paul with me everywhere. On medium difficulty it's not a hassle, just an inconvenience. Just remember where you've left it each time and be careful of explosions.
There is a new hidden area which the creators of this mod have added to the game in Dowd's mausoleum. After the door keeper opens the door for you, kill him. His hut has a medkit and a multitool, even though you don't need them for this achievement. From the door, go forward and to the right. Near some dumpsters you'll find an opening in the wall that contains the nanokey for the the chapel. Take it and go back to the chapel, which is to the left of the door. Follow @Jonex's solution for the achievement "Unearthed Horrors."
Once you find the ancient sword, you can drop Paul's corpse right there and then for the last time and the achievement will unlock. I dropped it inside the "coffin" that the sword was in, so do the same if you've dropped it somewhere else in the room and the achievement didn't pop.