STRAIN is a 32-level partial conversion released on September 14, 1997 by The Alpha Dog Alliance. It features nine new or modified monsters through the use of DeHackEd, and a new soundtrack written by David "Tolwyn" Shaw, Mark Klem, and Jon Landis.
The fist attack is now a quick, double punch combo, and the pistol has a faster rate of fire. The shotgun has edited graphics and new sound effects, rendering it into a sawed-off shotgun, as well as a faster rate of fire. The plasma rifle is replaced with the NFG, whose base damage is 7 points higher, resulting in a stronger projectile barrage. The BFG9000 is replaced with the Psychic Blaster, which shoots partially invisible BFG blasts that fly slowly and cause ordinary blast damage rather than the unusual kind exhibited by the BFG9000, but has a much quicker rate of fire. Unlike the Doom II cell weapons, the NFG and Psychic Blaster use different types of ammunition: cells for the NFG (with a maximum capacity of 50 instead of 300), and rockets for the Psychic Blaster, using three rockets per shot.
Usage of new tiles and DeHacked editing creates several new monsters for the WAD:
- Class 2 Imps, which move significantly faster and throw fireballs in a rapid manner, now replace the Arachnotrons. They have no visual distinction from normal Imps (which are also in the WAD), but have a higher-pitched alert noise. They will run in a player's face faster than can be seen and are hard to target due to their speed.
- Demons are now a deep red color and take slightly more damage (a super shotgun blast will kill it in one shot, however, assuming that all 20 pellets connect).
- Holobots, flying drones, replace the Revenant, gaining a Cacodemon-like hovering ability. They fire both types of the Revenant's missile, both of which travel at roughly half the normal speed and are easier to dodge. They can also be found stealthed much like the Spectre.
- Dopplegangers are Lost Souls which have blue and red flames with longer horns, and, in addition to the standard ramming attack, fire Imp fireballs.
- Cyberdemons are now half the normal size with higher-pitched alert and walking sounds. They take roughly four to six rockets to kill, but still fire normal rockets themselves, offsetting the comical idea of a miniature Cyberdemon.
- Replacing the Spider Mastermind are flying cubes known as Polydrones (taking graphics from the Icon of Sin's enemy spawn cubes). Their alert sound is a human voice saying "Got Him!". They fire the Spider Mastermind's chain shotgun.
- The Mancubus is replaced by the Demon Lord, a supercharged Baron of Hell with gray skin and green legs. They toss five green plasma balls simultaneously in a spread pattern.
- Certain zombie troopers fire off BFG9000 rounds in later levels. There is no visual distinction between regular and BFG troopers until they fire.
- Subverted Guards are replacements for chaingunners that have a new graphic set wearing the Marine helmet. The player sprite uses the same sprite set.
- Cacodemons, Hell Knights, and Barons of Hell fire their projectiles much faster.
- MAP01: Barracks by Bill McClendon
- MAP02: Outskirts by Bjorn Hermans
- MAP03: Downtown by Florian Helmberger
- MAP04: Industrial Zone by Adam Windsor
- MAP05: Depot by Jon Landis
- MAP06: Launch Control by Jon Landis
- MAP07: En Route by Adelusion
- MAP08: Hangar by Rich Johnston
- MAP09: Cargo Bay by Adelusion
- MAP10: Entryway by Charlie Patterson
- MAP11: Command Control by Bill McClendon
- MAP12: Power Station by Bill McClendon
- MAP13: Ruins by David Rotramel
- MAP14: Engineering by David Graves, Adelusion, Rich Johnston, and Arthur Chang
- MAP15: Subsidiary Power by Chainsaw
- MAP16: Living Quarters by Charlie Patterson and Eric Roberts
- MAP17: Promenade by Holger Nathrath and Arthur Chang
- MAP18: Relay Station by David Rotramel
- MAP19: Waste Processing by Darrell Esau
- MAP20: Reactor by David Davidson
- MAP21: Maintenance by Adelusion and Arthur Chang
- MAP22: Specimen Storage by Adelusion
- MAP23: Dispensary Alpha by Adelusion
- MAP24: Sub-Laboratory by Adelusion
- MAP25: Main Laboratory by Arthur Chang
- MAP26: Main Laboratory II by Holger Nathrath, Charlie Patterson, and Eric Roberts
- MAP27: Dispensary Beta by Andy Badorek
- MAP28: Unknown by Bjorn Hermans
- MAP29: Self-Destruct by Ron Allen
- MAP30: Boss by Jon Landis and Rich Johnston
MAP07: En Route is impossible to complete in source ports based on old versions of ZDoom (prior to 2.3.0), as the level relies on the occurrence of a spechits overflow in order for the exit to function properly, though new versions of ZDoom identify the map and automatically apply the required compatibility flag. There also problems with the DeHackEd patch when it is used with Doom Legacy. Doom Wad Station provides a fixed version of STRAIN, created by Zorcher in 2005, at the site's STRAIN page; it is compatible with ZDoom (version 2.0.63a, 2.0.96, or higher), Doom Legacy (version 1.42), and Skulltag (version 0.96c).