You are playing Nightmare mode.
Forced: After The Thing in the Depths enemy enters play, it attacks each player in turn order from the staging area (deal and resolve a shadow card for each attack).
Stage 3B gains: "Forced: At the beginning of the quest phase, if there are fewer locations in play than the number of players, discard cards from the top of the encounter deck until a location is discarded. Reveal that location.
Forced: At the end of the refresh phase, resolve the grapple keyword on each Tentacle enemy in play.
Begin with the standard quest deck and encounter deck for the The Thing in the Depths scenario.
Remove the following cards, in the specified quantities, from the standard encounter deck:
2x Corsair Infiltrator
1x Crew Quarters
1x Crushing Arm
1x Grasping Arm
1x Giant Arm
Then shuffle the encounter cards in this Nightmare Deck into the remainder of the standard The Thing in the Depths encounter deck.
Finally, flip this setup card over and place it next to the quest deck. Its effect remains active throughout the game, which is now ready to begin.
4
1
5
9 
Surge. Grapple (most enemies grappled).
Forced: After Slitering Arm engages a player, it makes an immediate attack against that player.
6
1
7
1
Grapple (fewest enemies grappled).
Forced: After Crashing Arm is detaced from a location, deal it 3 shadow cards.
Shadow: Attacking enemy makes an additional attack after this one.
When Revealed: For each location in play that is not grappled, reveal the top card of the encounter deck. If no cards are revealed by this effect, Deep Maelstrom gains Doomed 5. Cannot be canceled.
Shadow: Attacking enemy gets +1 for each enemy grappled with a location.