Abadox

Abadox: The Deadly Inner War is a Shoot'Em Up developed by Natsume for the Nintendo Entertainment System in 1989 and published in the US by Milton Bradley of all companies in 1990. The game's plot involves a galactic soldier's mission to Save the Princess, who was eaten by a Planet Eater named Parasitis. The game is most often compared to Life Force due to its organic theme, and is notorious for its horrendous difficulty.

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Tropes used in Abadox include:

Attack Its Weak Point: Most of the bosses have a "sweet spot" that needs to be shot.
Collapsing Lair: The final stage has no enemies; you must instead navigate your way out of Parasitis' rectum before the alien blows up.
Digital Bikini: That Princess you have to save? She's naked in the Japanese version.
Eldritch Abomination: Parasitis qualifies. Sure, it's primarily a Planet Eater, but speaking biologically this thing SHOULD NOT BE.
Eternal Engine: Stages 5 and 6 are what's left of the ship the Princess was on.
Evil Is Visceral: The game is full of this trope.
Eyes Do Not Belong There: Eyes are everywhere inside Parasitis.
Faceless Eye: One of the enemies in the first stage, and the miniboss of the penultimate stage.
Flaming Skulls: An enemy in the first stage.
Flunky Boss:
- The penultimate boss only attacks if it's flanking eyes are present. If the flanking eyes are destroyed, it's a sitting duck.
- The final boss has two minions attack from behind. This angle of approach makes them hard to take out.
Giant Enemy Crab: The miniboss of stage 4.
Gorn: One of the goriest games on the NES. Even the logo is covered in blood.
Hard Levels Easy Bosses: In almost any boss battle, there's one spot on the screen where you can sit and shoot all day without fear of getting hit yourself.
Heartbeat Soundtrack: Toward the end of stage 4 as you approach the Cilia Monster.
Humongous Mecha: The boss of stage 5 is a robot who's about five times the player character's size.
King Mook: The miniboss of stage 4 is a larger version of the crabs you encounter throughout the level.
Meaningless Lives: Losing all your lives only results in your score being reset. You don't even lose your checkpoint. However, since this means you'll have to go through the second half of a stage with no power-ups, this actually adds to the game's difficulty.
Mirror Boss: The miniboss of stage 5 consists of three robots who are about the same size as the player character and use the same weapon.
Nintendo Hard: The last few stages dip into Bullet Hell territory; without the tiny hitbox that makes most Bullet Hell games easier than they look.
Planet Eater: Parasitis, yep. The plot (or at least the Japanese original) even suggests that it intends to ultimately devour and adsorb the entire universe; Abadox is just its starting point.
Save the Princess: Yup, as it ever was in oldschool gaming.
Smashing Hallway Traps of Doom: Appears in the first part of stage 6. The single-piece crusher is almost as tall as what can be shown on screen.
Trial and Error Gameplay: Without sheer luck, there is no way you will dodge the hands in stage 2 or the phoenixes in stage 6 the first time you play.
Wall Master: During stage 2, hands will come out of the walls and attempt to kill you.
Womb Level: You're inside a Planet Eater for most of the game.