Maskim Xul is a three-level PWAD by Obsidian, designed for Boom-compatible source ports. Heavily influenced by the works of H.P. Lovecraft, it features extensive DeHackEd work to create new enemies, items, and weapons, as well as some emphasis on solving puzzles in addition to fighting demons. According to the text file, development began on 27 April, 2014. The first beta was released to the public on 31 October, 2017, and a "final release candidate" was made available on 7 December, however a number of further updates were made, and the WAD was eventually uploaded to the idgames archive on 6 April, 2018, followed by a later patch on 31 October, 2018. The WAD was named as one of the winners of the 2018 Cacowards.
- Lesser acolyte
- A floating dark mage in black robes, similar to the Disciples of D'Sparil from Heretic. Lesser acolytes replace the zombieman, and have 125 health. They attack by pausing in mid-air and launching a barrage of three imp fireballs in quick succession. They drop a demon heart on death when playing Map 3 (see new items).
- Greater acolyte
- A tougher variant of the lesser acolyte, distinguished by its brown robes and green symbol. They replace the shotgun guy and will attack by dealing rapid damage to the player while the remain in the acolyte's line of sight. They have 250 health, and drop a demon soul on death when playing Map 3.
- Acolyte of afrit
- This acolyte can be distinguished by the red trim on its black robes. It replaces the chaingunner and is tougher than the lesser and greater acolytes. It has 300 health, and attacks by launching a meteor that is capable of killing the player from 200 health and armor. The meteor deals splash damage, and the acolyte can damage itself with it.
- This tall, brown, horned demon moves slowly, and only possesses a sequence of 2 melee attacks, which can deal rapid heavy damage to the player. It replaces the spectre and has 500 health.
- Will 'o wisp
- This monster takes the appearance of a lost soul with green flames. It will not attack, but it is extremely fragile, with just 5 health, meaning it can be killed with a single shot from the pistol. Upon death, or upon touching the player, it will explode, releasing three baron of Hell projectiles in a short spread. This monster replaces the mancubus.
- Lord of heresy
- An orange, winged variant of the Hell knight/baron of Hell, which will attack with either a single projectile, or a spread of three. It is always throwing projectiles towards the player (when alerted), no matter if he is in plain sight or not, so it can accidentally hit other monsters and provoke monster infights. It replaces the cacodemon and has 1250 health.
- This light brown cacodemon variant replaces the baron of Hell and has 500 health. It attacks primarily with a bright green projectile that deals up to 56 damage and can be either unguided or homing, but will also launch a stream of six imp fireballs if its pain state is triggered.
- Although the arch-vile is not replaced in the WAD, a duplicate replaces the arachnotron.
- Damned soul
- A lost soul with grey flames. It has 50 health and will not charge the player but will explode upon death touching the player, dealing heavy damage to anything within the splash radius. It replaces the pain elemental.
- Turrets occur only in the first map, and are immobile and invincible. They fire slow-moving red projectiles which will instantly kill the player upon contact. They replace Commander Keen.
- This recurring mini-boss is encountered in the form of large mouths embedded into walls throughout the second level with the sprite itself - an immobile acolyte with green-trimmed black robes usually unseen. It has 2000 health, and attacks by continuously launching spreads of three knight/baron projectiles whilst the player is in the line of sight. If it takes damage, it will also launch a meteor identical to that of the acolyte of afrit. This monster replaces the spider mastermind and it is also immune to all blast damage, taking only direct hit damage from hits.
- Tentacles rise from the ground and are encountered throughout the third level. Tentacles are immobile and attack with the cacolich projectile. They have 250 health and replace red pillar with skull decoration.
- The final boss of the WAD, encountered in the third level. Shub-Niggurath has two forms. In its first form it is able to attack with either revenant missiles of huge spreads of knight/baron projectiles, as well as launching a barrage of three meteors if its pain state is triggered. In its second form, it will lose the knight/baron projectile flurry attack, though upon pain state it will summon monsters similar to the Icon of Sin instead of the meteor barrage. A DECORATE variant for ZDoom-based ports is included, due to the monster not functioning properly in GZDoom. Interestingly the health for the two forms is different in the DECORATE definition. In ZDoom ports, it will have 80000 health (30000 + 50000 for the respective stages), otherwise it will instead have 62500 health (25000 + 37500). The first stage replaces the cyberdemon, whilst the second stage replaces the Wolfenstein SS. The sprites used by the boss were created especially for the WAD by SuperCupcakeTactics.
- Dual pistols
- These replace the default pistol, and simply allow a doubled rate of fire.
- Hand of afrit
- This weapon technically replaces the chaingun, though it is analogous to the rocket launcher, and occupies the #5 slot. It is a magical attack allowing the player to launch the meteors used by the acolytes of afrit. Conversely, the original chaingun replaces the rocket launcher.
- Jackbombs replace the plasma rifle, and when thrown will have a short wind-up time before the Jack pops up and the box explodes. An explosion does up to 1000 points of damage and has a wide blast radius. The player can hold up to 20. Sometimes, a thrown box just disappears and deals no damage.
- Grimoire Excidium
- This evil spell book replaces the BFG9000. When the trigger is pulled, after a delay of 20 tics (approximately 0.571 seconds), the damage is calculated: 80 invisible tracer rays are emitted from the player in a cone-shaped volume (about 45° half-angle) in the direction the player was looking. Though extremely powerful, it uses the player's own health for ammo, so it must be used with caution.
Additionally, the shotgun pickup replaces the box of rockets, though the weapon itself is unchanged.
- Demon heart
- These are sometimes dropped by lesser acolytes, and are used as ammo for the hand of afrit, each counting for one shot. They replace the clip, whilst the original clip replaces the individual rocket ammo pickup.
- Demon soul
- These are sometimes dropped by greater acolytes, and are used as ammo for the hand of afrit, equivalent to five shots. They replace the shotgun.
- Roulette sphere
- This power-up replaces the evil eye decoration. It rapidly cycles through the appearance of the soulsphere, megasphere, invulnerability sphere, partial invisibility, and the berserk sphere, and will give the player whichever power-up the cycle was on at the time of pickup.
- The WAD itself and all its levels (save for MAP30) are named after supposed Ancient Sumerian phrases, quoted in various places on the Internet. "Maskim Xul" in particular is said to mean "Demon who lies in wait"; the name of the epilogue map, "Barra!", means "Begone!"