This guide will show you where to find every Mod (and Vault) in End of Abyss.
Mods are essentially the rings of End of Abyss, giving your character passive abilities when equipped. There are 10 in total, and 8 of them are found in Vaults. To open a Vault, you will need a Vault Key, which you get by defeating bosses. 5 of the 8 keys come from story-related bosses, and the other 3 from optional bosses. We will go into detail on where to find them all below.
Opening every Vault will unlock the 
Dungeon Crawler trophy.
None of them are missable, and they can all be found after finishing the main story.
Vault Keys
Vault Key #1
Obtained by defeating the Husk Cluster boss. This is the first story-related boss of the game.
Vault Key #2
Obtained by defeating the Husk Centipede boss. This is the second story-related boss of the game.
Vault Key #3
Obtained by defeating the Security Sentinel boss. This is one of the story-related bosses that you must defeat when looking for the 3 SCUs.
Vault Key #4
Obtained by defeating the Drowned Husk boss. This is one of the story-related bosses that you must defeat when looking for the 3 SCUs.
Vault Key #5
Obtained by defeating the Radiant Host boss. This is one of the story-related bosses that you must defeat when looking for the 3 SCUs.
Vault Key #6
Obtained by defeating the Husk Centipede a second time. This is one of the three optional bosses in the game. You can find it by climbing down into the Reactor Pool after draining it.
To drain it, you will have to press some switches and do a few other things over in the Waterworks area.
First, head to the room on the left of B3 and press the switch on the wall. You will need to have progressed far enough in the story to have the Grappling Hook.
After that, head to the right side of the circular structure near the top of B3 in the Waterworks and press the switch to drain some of the water.
Next, activate the pump by interacting with the console in the room on the eastern side of B3 to drain the water in that area.
Then head out through the door to the left of the pump console and climb down the ladder. Continue along this path and head north to the dead end. Here, use an Explosive Charge to destroy the wall and clear the blockage.
Finally, head through the door in the top right corner of B3 and follow this path to the end, where you will find the switch that drains the Reactor Pool. Along the way, you will have to open an Energy Node door and survive an ambush.
Vault Key #7
Obtained by defeating the Brood Keeper boss. This is an optional boss found in the Ventilation Tunnels area. First, you must destroy the weak wall to the right of the easternmost entrance to the Tunnels from Factory East. You can do this with the Explosive Charges, which are obtained during the main story. On the other side of the wall, continue right until you eventually reach F2 of the Ventilation Tunnels. Here, head around the upper catwalk and pull down the ladder to reach F3. Then head all the way right and go up the stairs for an ambush encounter. After defeating the enemies, grab the Ventilation Maintenance Key.
With the Maintenance Key, head back to the bottom left part of F3 and use the Grappling Hook to pull down the ladder. Climb up, open the door with the key and press the switch inside.
After pressing the switch, head back down to F1 and climb the ladder in the circular area to reach the boss.
Vault Key #8
Obtained by defeating the Morgue Dweller boss. This is an optional boss found at the end of the Infirmary, an optional late-game area. To get there, you must first get the Infirmary Key from the Lower Levels area (the final area of the game). You can find it by burning the Parasitic Spire in the right side labs on F1 (you will need the Incinerator, a story-related weapon obtained in this same area).
Once you have the key, use it to access the lift to the right of the Fast Travel Shuttle in the Habitation Sector to get to the Infirmary. This is a fairly short area, and the boss can be found at the end. To reach it, first grab the Restricted Access Keycard from the small room on F1. Then use it to get to B1 (through the door at the top of the right side corridor). You will then find the boss on the right side of this floor.
Mods (and Vaults)
Vitality Enhancer Mk. I
You start the game with this Mod.
Radiation Protection
This Mod is essentially story-related, since you need it to survive the critical radiation zones in the Containment Sector. To get it, head left from the safe room at the start of the Containment Sector to reach the Factory East area. Here, pick up the Basement Keycard and press the switch to extend the bridge.
Cross the bridge and continue left until you see a ladder you can climb down. At the bottom, head right to reach the Basement area. When you get to an area with some strange growths, head down the metal stairs and then the ladder (before going down the stairs, you can also go right to open a shortcut back to where you found the Basement Keycard, which is recommended since you will have to survive an ambush down below). At the bottom of the ladder, move towards the screen and use the Basement Keycard to open the door. Survive the ambush inside and head through the door on the left to get the Radiation Protection Suit Mod.
Equipment Belt
Found inside the Vault at the end of the hall in Habitation Sector > F1.
Vitality Enhancer Mk. II
This one can only be obtained after completing the main objective where you have to find the 3 SCUs. Then head to Communication Center > F2 and take the lift on the right side down to the lower level. Here, head through the door on your right, then go down the trap door to find the Mod in the Vault below.
Heal Amplifier
Found in the Vault in the top left corner of Reactor > B4.
Blast Resistance
Found in the Vault on the left side of Factory West > F2. You will have to blow a hole through the wall with an Explosive Charge.
Acid Neutralizer
First, head to the room on the left of Waterworks > B3 and press the switch on the wall. You will need to have progressed far enough in the story to have the Grappling Hook.
Next, activate the pump by interacting with the console in the room on the eastern side of B3 to drain the water in that area.
Then head out through the bottom door and look for a ladder leading down into the now drained pit. Climb down and head right to find the Mod in a Vault.
Proximity Sensor
Burn down the Parasitic Growth in the bottom right corner of Containment Sector > F1 with the Incinerator, the flamethrower you get in the final area of the main story. Then climb down the ladder it reveals to find the Mod in a Vault.
Auto-Defibrillator
Found in the Vault on the left side of Infirmary > B1. To get to the Infirmary, first burn the Parasitic Spire in the Lower Levels area to get the Infirmary Key. Then use it to open the lift to the right of the Habitation Sector's Fast Travel Shuttle.
Experimental Core
Found in the Vault in Lower Levels > F1. To reach it, you will need the Level 3 Access card.
First, you need to get the Level 2 Access card, which can also be found in the Lower Levels. Burn down the wall on the right side of F1 with the Incinerator (the story-related flamethrower, also obtained in this area) and head into the large room. Here, look for a trap door in the middle and climb down. After coming out the other end of the vents, head right and loot the Level 2 card from the corpse.
Once you have the Level 2 card, you can get the Level 3 card. For this, head through the door just south of the wall you have to burn down during the story on F1. Head up the stairs inside, go up through the corridor, exit left and then left again. You will find the Level 3 card in the top left corner of this room.
Once you have the Level 3 Access card, head through the door on the right side of F1 and open the Vault.
