Your Isaac run is being invaded by Rogue Glitch Ultra.
Glitch chests. One waits in the first room of every run, and more drop from glitched rooms. Touch one and it bursts open with RGU's chest sound, leaving a Rogue Glitch Ultra item on a pedestal.
19 RGU items as Isaac items, with RGU's own icons in its rarity colours: Gatling Gun (spins up while you hold fire), Electric Gun, Squirt Gun, Homing, Piercing, Split and Exploding Shots, Glitch Boomerang, Spiked Ball, Viral Infection, Vampiric Instincts, Pixel Heart, Corrupt Data, plus the actives Missile Barrage, Item Reroll, Glitch Laser and Glitch Reaper.
Glitched rooms. Some rooms glitch as you walk in. RGU's glitch texture tears across the screen, enemies flicker and get tougher, and the room plays RGU's combat track for your floor: Megabyte Forest, Variable Desert, Sea of Data, Realm of Decay, Tower of Algorithms or The Creator's Domain. Clear it for a glitch chest.
Single player. Needs The Binding of Isaac: Rebirth (Repentance or Repentance+) and Rogue Glitch Ultra. All of RGU's art, sound and music is converted from your own copy when Melty sets it up; nothing from either game is shipped.