The Fiend bears resemblance to a corpse-like demon, from its overall jagged and withered humanoid body structure, to its taloned face and elongated head. Pin-prick pupils glare out from beneath two empty dark sockets. Its thighs are fused together by flesh, and below its knees, its legs are short, stubby, and end in points instead of feet. The Fiend is surrounded at all times by an aura of power in the form of a hazy purple fog.
The Fiend possesses supernatural, telekinetic powers that it uses in a variety of sinister ways.
INTELLIGENCE: Described as the most intelligent monster, the Fiend is actually not that much smarter than the Brute or the Hunter, it simply seems to understand that it has been tricked when it comes to investigate a noise but finds nothing, and will not linger in one spot for as long as the other monsters do (unless it has seen the player enter the area). If the Fiend knows the player is around, even if it has seen them enter a room with no exits, it may linger for a bit but will not check every hiding place in the room as it has been claimed to do.
However, the Fiend's persistence in searching a relatively large but narrowly focused area, coupled with its electric distortions, makes it arguably the most difficult monster to get away from if the player hides in areas such as the Upper Deck where the Life Raft is, and in the Engine Room. This is where the player's fear will actively work against them, as the impulse to remain hidden while the lights are perpetually flickering will keep them where they are, potentially permanently as they wait for an opening to get away, one that may already be present yet does not appear to be.
SPEED: The Fiend is the slowest monster in a chase, but its speed does not change much between chase mode and patrol mode, making it the fastest monster when not chasing the player. The Fiend can also "climb" up through the holes in the cargo hold via levitating itself up or down as needed, making it just as quick in that area of the ship as any other.
STRENGTH: The Fiend is the weakest monster physically, however this should not be taken for granted. Its telekinetic powers allow it to easily lift the player, as well as rupture locker doors and rip them off their hinges. It is limited in its ability to open locked doors in that it must first unlock them before it can open them, a fact that can be exploited if the player re-locks the door before the Fiend opens it.
SPECIAL ABILITIES: The Fiend possesses the largest number of special abilities, all of which have to do with its telekinetic powers. It can paralyze a player that remains within its line of sight for too long (represented by purple tendrils crawling up from the edges of the player's vision), an effect that stops your movement for roughly 2 seconds, and that will kill you if it happens 3 times within 10 second intervals. The Fiend is also able to shut and "lock" every door within a large radius with an aura of purple energy similar to the aura around itself, which can easily trap a player attempting to escape a room or a linear corridor. The Fiend causes a disturbance in certain electrical systems when it passes by them (see below).
The Fiend is the only monster that does not make footsteps, meaning the player must rely on sight more than sound to detect it:
FLICKERING LIGHTS: The Fiend's presence, whether it intentionally does this or not, causes all electricity-based lights to short out within a fairly large radius around it, causing them to flicker more and more intensely the closer it is to them, until finally shorting out completely while in its presence (though they will turn back on when it leaves the area). This also affects the player's flashlight, making it a vital tool for detecting the Fiend's presence in areas that lack sufficient electric-based light (such as the cargo hold).
DEMONIC SOUNDS: Breathy, seething noises that can only be heard at a relatively close range. These sounds appear to have character, and range from curiosity to irritation.
WARPING/HUMMING SOUND: The Fiend's power can not just be seen, but it can be heard as well. Typically this requires that the Fiend be only several feet from you, but it can be heard through floors and ceilings. It's also an important sound to listen for while in the cargo hold, as it means that the Fiend is very close.
- The key to defeating the Fiend relies on your ability to be constantly in motion, ping-ponging back and forth from the farthest sides of the ship. Search quickly through as many rooms as you can, but leave as soon as you notice any lights starting to flicker (preferably from a distance). It is always better to run from the Fiend than to hide, unless you have no other choice; hiding disorients the player, as once the flickering lights are "on top" of you, determining where the Fiend is exactly from that point on can be difficult.
- It is important to note and be able to recognize the distinction between the two kinds of flickering lights that can be found in the game; persistently flickering lights that just barely change the brightness of the area around them, which is an ambient effect, and "heavily" flickering lights that make the area very dark every time they flash, which is caused by the Fiend.
- The cargo hold is the most difficult area to search through, as there are many ways in and out of it that the Fiend may use, making every turn a gamble as to whether or not you'll run in to it (assuming it's nearby). You should be able to find all the items you need for at least one of the escape routes without ever going to the cargo hold, so save it as a last resort (unless you manage to put some distance between you and the Fiend and are confident you have time).
- Try to avoid running down narrow, linear corridors as much as possible if the Fiend is chasing you. This is where the Fiend's abilities as outlined above shine, and is the counterweight to its general slowness - it doesn't need to outrun you, it just needs to trap you.
- If the player is in an unlit area such as the cargo hold, they're advised to "tap" their flashlight (activate it and deactivate it) to check for the Fiend's presence. Regularly doing this trick can save the player's life in such areas.
- The Fiend is immune to fire and is notably the only monster unaffected by pit traps (it does not walk).
- The Fiend is vulnerable to flare guns, fire extinguishers, and steam.
- Close & lock the doors if possible, The Fiend take a long time to open a locked door, which gives you enough time to hide or run.
|In Markiplier's playthrough of Monstrum, a ghostly disembodied head appeared for less than a second when he exited out of the journal, which some speculated had something to do with the as-of-then unimplemented third monster. Nothing like this has been found in-game; this was simply an editing trick.|
|The Fiend's eyes are always trained on you. This is the result of a clever illusion, where its pupils are set back into its head.|
|The Fiend performs a special animation after it successfully paralyzes the player. For obvious reasons this goes unnoticed in a normal playthrough, but can be seen clearly when using a loophole.|
|The Brute • The Hunter • The Fiend|