On my YouTube channel I've created a gameplay walkthrough video covering chapter 12 of this game.
Chapter 12: Dead Space - In this video Isaac, now on Aegis VII, has to return the Marker to the excavation site in hopes to end the horror brought on by the Hive Mind and its Necromorphs. After fighting our way through the supply depot, transfer junction, and storage chamber, we make it to the excavation site.
*** Spoiler - click to reveal ***When we reunite the Marker with the pedestal, the gravity tethers release the payload from the planet crack pulled caused by the Ishimura, and we receive an impact imminent warning. When we try, we receive an error message. Daniels returns and we learn the truth about Nichole and find out the fate of Dr. Cross. With the danger of the impact still present, we rush back to the ship to intercept Daniels, but run into the Hive Mind (final boss). After a fight with this massive, tentacle covered monstrosity, we are able to make our escape.
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After the fight with the Hive Mind you should unlock the Mindless Prey achievement (49:40 in the chapter video).