Gjenganger  are the ghosts of Vikings.
While Ray Stantz was telling Raphael an anecdote about his nephew and Peter Venkman in a snowy forest in an alternate dimension, an invisible Gjenganger picked up Ray and shoved him into a tree. Ray instructed Raphael to jump up and reach for his Particle Thrower. Raphael got the Proton Pack on demanded the ghost to show himself and stated he wasn't in the mood for games. The Gjeganger didn't care, revealed himself, and knocked Raphael out.
Ray and Raphael's bodies were possessed and they underwent a forced out-of-body experience. Their consciousnesses levitated nearby. Ray observed the Gjenganger and determined they stumbled upon an afterlife and their bodies were hijacked so the Gjenganger could experience the sensation of physical combat again. Raphael regained lucidity and asked what was going on. Ray got an idea and believed a show of bravado would get the ghosts to abandon their bodies. Ray got the Gjenganger's attention. He apologized for intruding on this dimension but asked for the bodies back. The Gjenganger laughed. Raphael opted for a direct approach and rushed their bodies but he bounced off a shield. Ray instructed Ray to push all the fight he had into the forefront of his conscious mind and use it like a battering ram. Together, they forced a Gjenganger named Nils out of Ray's body and occupied it. Ray became distracted by Raphael's memories and feelings of loneliness. The other possessor swung his axe. Luckily, Raphael noticed and raised their shield. Raphael refused to let Ray shoot his body to liberate it and took control. He hoped Ray's joints weren't as old as he looked.
Raphael was confident since the Gjenganger had no sense of misdirection but his body was too tough to beat with Ray's. Raphael got mad and complained he was going to be stuck in Ray's body forever. Ray told him he was so negative then got an idea. He instructed Raphael to use the Slime Spritzer on the right hand side of his belt and spray both his and Raphael's body at the same time. Ray gambled the slime would interact with Raphael the same way it would with a possessor since he was so angry. The Gjenganger was ejected from Raphael's body and Raphael was ejected from Ray's body back into his own. Raphael wondered how that worked but the Gjengagner swarmed them. The Remote Portal Access Band was charged and Ray opened a portal. They jumped through in time and none were able to follow.
Based on Mythology
Gjenganger are corporeal ghosts from Scandinavian lore. They are often spirits of the murdered, those who died by suicide, or murderers.
- During his undergraduate years in college, studying abroad, Egon Spengler learned the Swedish language while he was researching Gjenganger. 
- On Cover A of Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #3, the Gjenganger on the right appears to be visually based on Thor and in the lower right corner, the hammer appears to be Thor's mythical Mjolnir.
- Teenage Mutant Ninja Turtles/Ghostbusters Volume 2
- ↑ Ray Stantz (2017). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #1" (2017) (Comic p.6). Ray Stantz says: "Well, in short... we were set upon by a group of Gjenganger"
- ↑ Egon Spengler (2013). IDW Comics- "Ghostbusters Volume 2 Issue #6" (2013) (Comic p.2). Egon Spengler says: "I know what he said. I learned Swedish when I was researching Gjenganger as an undergrad."
- ↑ Narrator (2017). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #5" (2017) (Comic What Came Before Page). Narrator says: "RAPHAEL and RAY STANTZ manage to de-possess Raphael's body and restore him to his rightful place and escape the frozen forest full of vengeful viking ghosts..."