Interdimensional Horror

 is the boss of The Marine's past.

Behavior
Interdimensional Horror is mostly stationary but has the ability to teleport. It has two bullet-based attacks:
 * Fires volleys of bullets in all directions, then releases a ring of bullets that must be dodge rolled over. The ring of bullets travels faster than the volleys of bullets and overlaps them.
 * Fires multiple wriggling lines of bullets towards the player.

After teleporting a few times, Interdimensional Horror will summon many enemies who move towards the player slowly and periodically shoot bullets while predicting the player's movement.

During the fight, the ground may temporarily become poisonous in certain areas, limiting movement space. This poison inflicts damage much faster than other poison in the Gungeon.

There are 4 marine NPCs in the bottom corners of the fight hiding behind cover, who will shoot at both Interdimensional Horror and the minions it summons.

Trivia

 * The Ammonomicon entry for the Interdimensional Horror describes the plot of the Doom series of games, where the United Aerospace Corporation developed teleportation technology that instead opened a gateway to Hell. The series is famous for, among other things, starting the trend of protagonists in shooters being an unnamed space marine.
 * The Primerdyne corporation in the Ammonomicon entry is reminiscent of Cyberdyne, the corporation who developed the technology leading to the fall of mankind at the hands of artificial intelligence in the Terminator franchise.
 * The minions it summons resemble imps from the Doom series and are referred to as such in the game files.
 * Interdimensional Horror's minions appear to be among the only 3 enemies in the game to visibly bleed upon receiving damage, the others being Dr. Wolfenclaw (when killed) and the humans summoned by The Last Human.