Abyssal Tentacle

 is a gun that fires a tentacle which homes in on and grips enemies, ignoring obstacles.

Trivia

 * This gun may be a reference to Day of the Tentacle, in which a sentient tentacle of the same color scheme becomes engrossed in the prospect of world domination.
 * The synergy is a reference to the Metal Gear Solid character, Decoy Octopus and the purple, tentacled Legend of Zelda enemies of the same name.
 * It is also a reference to Dave Crooks, one of Enter the Gungeon's developers, whose handle is DecoyOctorok.
 * The gun could also be a reference to the classic MMORPG RuneScape, where the Abyssal Tentacle is a power whip-like weapon.
 * The Synergy is a reference to the Lovecraftian Horror genre, which is also sometimes called "Cosmic Horror." Cosmic horror deals with malevolent, godlike aliens affecting our reality in various unpleasant ways, including tentacles coming out of black holes.