The Bronx Spook is a large bestial ghost with vague insect-like traits.
Cait Banner, Evan Torres, and Zoe Zawadzki opened a portal to another dimension with the Ghostbusters' Interspatial Teleportation Unit as part of a tour of the Firehouse for their new classmate Garrett Parker. Evan assured Garrett it was safe to put his hand through the portal. On the other side, in the other dimension, the portal opened in the evening outside of the Kenneth P. Higgins Institute. The Bronx Spook was attracted to the portal and surfaced from underground. It grabbed Garrett's hand with a tentacle and attempted to pull him through the portal. Evan and Cait tried in vain to keep Garrett still. Zoe hit the red button on the console and shut the portal. Part of the Bronx spook's limb was severed and its ectoplasm went on Evan, Cait, and Garrett. The ghost's P.K.E. was split between the two dimensions and thus, existed in both places.  The Bronx spook roared then heard a voice commenting on how it could sense the spook but it wasn't where it was. The spook came closer to a ghost on the reflection of a store front across the institute. The ghost remarked the Bronx spook seemed to exist simultaneously in more than one dimension. The ghost found it cool then wondered what was going to happen when it tried to pull itself together and suggested they find out.
Enough of the ghost's spectral mass made it to the other side for it to be tethered to both dimensions. As a result, when it began to reform itself, it drew on the power from the psychokinetic energy between dimensions and the Manhattan Ley Lines. This in turn caused the frequencies of both dimensions to go into sync and they overlapped.  The merging was unstable. The most recognizable spatial anomaly caused by the overlap were two Statues of Liberty side by side of each other. As the days passed, the overlap turned into a worldwide phenomenon but the strongest effects were still felt in New York.
Ray Stantz, Egon Spengler, and Kylie Griffin concluded the problem was three fold. First, they had to sever the ghost's connection to the Manhattan network of Ley Lines. To do this, they would have to douse the Manhattan network with positively charged Psychomagnotheric Slime. They decided to harvest the slime found under Coney Island recently to cover the Manhattan network. Second, they had to fully draw the ghost into their dimension and trap it. They extracted the ghost's ectoplasmic residue from the 101 pilot team and planned to calibrate a trap specifically to pull in the ghost. Third, they had to seal the breach it would emerge from with a controlled explosion. If the plan failed, it was estimated the world would be overrun by a massive influx of ghosts then the misalignment of the dimensional frequencies would shake the world apart at the molecular level inside of six months. If the plan worked, they believed everything would return to its normal frequency and the dimensions would untether. Abby Yates suggested they re-purpose her team's Containment Unit to capture the ghost with. They quickly dismantled it and quickly constructed a prototype vehicle referred to as the Ultimate Mobile Trap. It held a larger, more powerful trap. Egon then tuned the trap to the overlapping frequency of the other dimension to grab the ghost. Kylie suggested two teams of Ghostbusters should be on standby on two roofs, within pack range, to wrangle the ghost out of the breach in case it resisted the trap's pull.
The Ghostbusters mobilized nearly all of their 101 Cadets and made them their duly deputized agents. One group of 28 cadets was overseen by Ray at Washington Square Arch, another group of 28 were overseen by Erin Gilbert outside a department store chain, and a group of 24 were overseen by Patty Tolan outside of Central Park. The cadets were armed with maps of the Ley Line network, Ecto Goggles to see the Ley Lines, and various types of equipment built to spray the positively charged Psychomagnotheric Slime. The neutralization of the Ley Lines was a success and the ambient psychokinetic energy levels dropped. Jillian Holtzmann and Abby took off in the Ultimate Mobile Trap and parked in Times Square's Duffy Square then opened the trap. After a moment, the ghost manifested and stopped halfway through the breach. The Ghostbusters opened fire and fully removed it. The ghost swiped at the teams in anger but the trap worked and it was captured. However, the breach remained. Erin realized they needed a catalyst to close the breach and start the reaction that would separate their dimensions on the molecular level. She suggested they Cross the Streams. Ray objected and so did Egon, but he knew they had little other choice. They came very close to full protonic reversal but shaking up the fabric of reality worked and the two dimensions were successfully split back into their normal states.
- The ghost is visually inspired by unused concept art of a Tentacle Creature done by artist Constantine Sekeris for the Ghostbusters (2016 Movie).
- Ghostbusters 101
- Ghostbusters Crossing Over
- Narrator (2017). IDW Comics- "Ghostbusters 101 #3" (2017) (Comic What Came Before page). Narrator says: "But the thing is, the ghost's energy is now split across the two universes. It exists in both places... and it wants to be whole again."
- Egon Spengler (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.3). Egon Spengler says: "If the ghost was trying to reform itself, it could - in theory - be able to draw enough power from the psychokinetic energy that exists between dimensions to cause our current overlap situation."
- 80-C Memo (2018). IDW Comics- "Ghostbusters Crossing Over Issue #1" (2018) (Comic p.22). 80-C Memo reads: "Classification: Class 5, enhanced."
- 80-C Memo (2018). IDW Comics- "Ghostbusters Crossing Over Issue #1" (2018) (Comic p.22). 80-C Memo reads: "PKE Reading: 15191519.05."
- Art of Constantine Tentacle Creature 3D Concept Rough 10/7/15
- Narrator (2017). IDW Comics- "Ghostbusters 101 #3" (2017) (Comic What Came Before page). Narrator says: "And then, of course, the open portal ruffles the psychokinetic energy that insulates the dimensional wall, and attracts a ghost."
- Ray Stantz (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.3). Ray Stantz says: "Any trace of the entity that attacked them when they first opened the door to your dimension."
- Egon Spengler (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.4). Egon Spengler says: "From what you all described, the dimensional door shut with enough of the ghost's spectral mass on this side of the gate to tether it to both dimensions."
- Ray Stantz (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.10). Ray Stantz says: "We need to sever the ghost from the Ley Lines it's drawing power from, then draw if fully to this dimension..."
- Ray Stantz (2017). IDW Comics- "Ghostbusters 101 #5" (2017) (Comic p.18). Ray Stantz says: "We need to disrupt the power of the Manhattan area Ley Lines, unstick an entity that's been trapped between our two dimensions and is bringing our frequencies into sync and then seal the breach behind it to keep from a massive influx of extra-physical entities."
- 80-C Memo (2018). IDW Comics- "Ghostbusters Crossing Over Issue #1" (2018) (Comic p.22). 80-C Memo reads: "The gate was powered down as an entity attacked, causing it to be trapped between two dimensions."