The Invasion

The Invasion is a 1997 partial conversion for The Ultimate Doom which replaces all four episodes. It was created by Paul Fleschute (Moe) and it includes a wide variety of changes, including new enemies through DeHackEd, many new textures and sprites, new music, and a new storyline which pits the player against the Aldorian alien race as they attack several Starbases owned by the USEC. Through its many custom graphics and enemies, the WAD manages to create a peculiar atmosphere that still remains unique to this day.

Monsters and other changes

 * The demon is now dark green in color, and its blood is green also.
 * The lost soul is replaced by a a small floating drone that however behaves much the same. It, and other robotic enemies like it, have a unique pain sound and do not bleed when shot.
 * The cacodemon is replaced by a floating, robotic drone that is partially invisible at all times, much like the Spectre.
 * The baron of hell is replaced by bulky, cylindrical security drone that fires the same projectile from a cannon located on the upper side of its body. Said projectile, while looking the same, has a different firing sound effect.
 * The spider mastermind is replaced by a huge robot wielding two chainguns.
 * Several props and pickups have been changed. For instance, the soulsphere is now a large green medikit called "Superhealth", the invulnerability sphere is now some sort of technological device, and the coloured skull keys have been replaced by coloured "chips", serving the same purpose.

Levels
The Rock: Project: Jupiter: Planet Hell: The Final Frontier:
 * E1M1: Launch Command
 * E1M2: Storage Area
 * E1M3: The Infirmary (Exit to secret level)
 * E1M4: Astoroid Mining Project
 * E1M5: The Caves
 * E1M6: Shipping Area
 * E1M7: The Observatory
 * E1M8: Command Station
 * E1M9: Central Processing (secret level)
 * E2M1: Security
 * E2M2: Containment Area
 * E2M3: Refinery
 * E2M4: Central Processing
 * E2M5: Central Command (Exit to secret level)
 * E2M6: Sickbay
 * E2M7: Power Control
 * E2M8: Terror
 * E2M9: Nuclear Plant (secret level)
 * E3M1: The Brigs
 * E3M2: City of Takira
 * E3M3: Temple of Doom
 * E3M4: Threshold of Hell
 * E3M5: Chaos
 * E3M6: Deep Beneath (Exit to secret level)
 * E3M7: Mt. Eldanar
 * E3M8: Death Domain
 * E3M9: Forbidden Temple (secret level)
 * E4M1: Fear
 * E4M2: The Jefferies Tubes (Exit to secret level)
 * E4M3: Perfect Hatred
 * E4M4: Cargo Hold
 * E4M5: Engine Control
 * E4M6: Detention Level
 * E4M7: Circle of Death
 * E4M8: Mission: Impossible
 * E4M9: Halls of Hell (secret level)