Xenocide

Xenocide is a fictional boomer shooter that is present as a recurring Easter egg in NINE100 Studios and DECA games. The game is a parody of old school corridor shooters. 
==History==
Xenocide debuted as a playable demo in In Decay as an Easter egg. It can be played by rebooting an old arcade machine in Cybertron Collectibles. The only level available is a so-called demo level, where the player uses an anachronistic boomstick while fighting off demons as they navigate through the level. The level was a time trial with an in-game leaderboard, prompting a small community to form around speed running the demo level.
The game was referenced multiple times throughout the In Decay franchise, but was never playable again. It would however make a reappearance in DECA's Undefined, which notably was not published by NCS. This time, it's again playable on a retro arcade machine, but this time is more fleshed out. There are 3 stages, 4 weapon types, and it caps off with a boss fight against the "Mother-of-All-Demons". While each stage would take a couple minutes each at most, the game was designed with the speed running community in mind, offering a lot of side objectives like collectibles, score awarded for killing in a certain way, and intentional skips.
This version of Xenocide, dubbed Xenocide FG by the community (officially meaning "Finished Good", but a community inside joke referring to "Fucking Good"), would reappear more often after this. However, instead of playable as on an arcade in-game, it was mostly accessible through main menu secrets or something similar. 
The last time Xenocide would appear as a modified version, was as a secret game mode in Neo Cyberscape. However, the game is presented as a glitched memory, where the player model is still in the art style of the main game and uses the same mechanics. The stages are still the same as in Xenocide FG, with the only difference being that the player instead finds themselves in the 3D rendered world of the 2D pixelated world, fighting off the sprites of Demons.