The Grundel  is a malicious ghost who is always looking for a willing child to manipulate into doing bad things. However in order for his power to work, the child has to willingly let him in to their house or accept a handshake from him in person.  Should the child continue to do bad things under his influence, then he or she will turn into a Grundel after awhile. Once the child has made a full transformation, the Grundel will then leave that child to find another.    
The Grundel easily gets a young boy named Alec Meredith to fall under his influence. While Alec is at first nothing more than aggressive and willing to steal, his crimes become dangerous when he messes up the Ghostbusters' Proton Packs while at an assembly (causing them to nearly burn the whole gym down). He also makes Alec steal a wallet while he is on the bus. After possessing Alec for a while, the Grundel then decides to target his brother Lee. The Grundel has Alec lure his brother to an old warehouse, upon entering Lee discovers that his brother has nearly completed his transformation into a Grundel. The Grundel then attempts to possess Lee, but Lee refuses to let him in. The Ghostbusters show up, yet they cannot attack the Grundel as he is linked to Alec and harming him would also harm the boy.  Alec and the Grundel chase Lee in order to make him allow the Grundel in. When Lee falls through the floor and holds on for dear life, Alec says that only the Grundel can help him and that he must accept the Grundel in order for him to take his hand. Peter Venkman however convinces Alec to not let his brother fall and Alec helps his brother as Peter wrestles with the Grundel. With Alec helping his brother, his bond with the Grundel is broken and the Ghostbusters are able to successfully capture him. 
This however would not be the end of the Grundel. 10 years later, a second Grundel appeared and started following in the footsteps of the first.  This Grundel turned out to be a boy named Jack, an old friend of Ghostbuster, Kylie Griffin. Kylie and Roland interrogated the first Grundel about what happened with Jack and learned from the Grundel that Jack had not reverted back to his original form as his strength of will- possibly inspired by his friendship with Kylie- forced the Grundel to place him in a special cocoon so that he could 'develop' properly, the cocoon only completing the transformation recently (Most likely the transformation would have been quicker if the Grundel had been there to 'nurture' it).  The Grundel was able to influence the nearby brother of Roland, Casey, to release him from the Containment Unit. Immediately after his escape, the Grundel teamed up with Jack and went after Casey in an attempt to trap Kylie. The Ghostbusters got there just as Casey had transformed. While the rest of the team went after Jack and Casey, Kylie stayed behind and faced the first Grundel herself. Though she was unable to beat him, Roland came in and caught the Grundel in a Proton Stream, giving Kylie the time she needed to set the Trap and re-capture the escaped ghost. With the Grundel recaptured, Casey and Jack was freed from his spell and is happily reunited with Kylie.
Before the Tunguska Blast of 1909 took place, John Horace Tobin was called to look into claims of a creature that had been preying on the children of an East German village. Tobin encountered a Grundel.
When two teams of Ghostbusters captured Proteus in their Proton Streams, the god was forced to show them the future. Since eight men captured him at once, he had to show eight futures at once. One of the futures gleaned was of a Grundel.
The Ghostbusters encountered Grundels on multiple occasions. They discovered a Grundel's main goal was to create more its kind and preyed on children. It ingratiated itself with a child, won their confidence, isolated them by encouraging them to commit sinful acts, including theft, gluttony, and violence, formed a parasitic bond, fed on their soul while replacing it with its own foul essence.  Eventually, the child died and rose as a new Grundel then the cycle repeated itself. The Ghostbusters were unable to trap a Grundel when it was linked to a child but discovered it was extremely photosensitive and was weakened by bright lights. However, they determined the most efficient method to separate the Grundel was to use positively charged Psychomagnotheric Slime on the host child. 
Jack, the Second GrundelEdit
See Jack (Grundel) article.
Like the Boogieman, the Grundel has a special connection to one of the Ghostbusters, Kylie Griffin. When she was a child, the Grundel attempted many times to influence her to come out, however she wouldn't come. The Grundel instead decided to target her friend Jack. She had been playing at her friend's house on a dark and stormy night when the Grundel came by the house. While Kylie heard the creatures calls to come out and play, she still refused to answer it. She instead ran to find her friend Jack in the house. However Jack did choose to answer the Grundel's call and Kylie caught him just as he was opening the window. Her last sight of him was his face transforming into a Grundel face. Jack apparently ran off into the night, leaving a terrified Kylie alone.
Kylie would confront her opponent years later after Jack resurfaced as a Grundel. She then learned that the first Grundel had kept Jack in a cocoon all these years and kept him from transforming back to his original state. When the Grundel managed to escape the Containment Unit, he went after Roland's brother Casey in order to draw out Kylie. While the other Ghostbusters went after Jack and Casey, the Grundel personally confronted Kylie. He no longer had any interest in making her into a Grundel, he just wanted to see her cower in fear. The Grundel proved more than a match for Kylie, but Roland managed to find them and catch the Grundel in a proton beam. Kylie then captured her former tormentor in the ghost trap and the Grundel was returned to the Containment Unit.
The Grundel's only real power appears to be his power of influence over a child's heart. When the Grundel gets a child to answer his call, he feeds the aggression in their heart and cause them to act like bad children. After a while of misbehaving, the Grundel's power grows and the child transforms into a Grundel. Although it usually takes time for the transformation to begin, in Extreme Ghostbusters, a child's face would instantly transform into a Grundel face while the rest of the body would change over time.
This aside, the Grundel also has the ability to travel amongst the shadows, entering and leaving where ever they may be. The only other possible power he seems to possess is his influence to get kids to come out. Though this is probably a mere skill rather than a power. A number of children who fall under his influence act as though they are in a trance, though this may just be sleepiness as he usually tries to get them while they are asleep. However when Alec became a Grundel the original apparently told him how to blast fireballs from his fingers.
In terms of weakness, the Grundel only seems to be weak when he has no children under his influence. When a child starts to transform, he becomes linked with the child, which is something the Ghostbusters had to be careful of during their first encounter with him. If a child refuses to listen to him or starts to go against him, he is an open target to the Ghostbusters. The Grundel also (like many ghosts) seems to be warded off by light, though whether he is truly weak against it or just tries to avoid being spotted is unknown.
The Grundel loves nothing more than to taunt children into doing dirty deeds. The Grundel seems to have great determination as he has been known to focus all his efforts on certain kids who do not answer his call, in the sense of Kylie. While the Grundel loves to find children who are willing to be bad, he seems to really enjoy pushing good children to do bad things. He also seems to possess some intelligence, as he chooses some tactical plans to affect the kids he cannot influence, such as attempting to influence the ones close to them. He also knows how to influence people while making it seem like he is merely in a conversation, as he was able to convince Roland's brother Casey to release him by saying "Release me now... let me free," though he had been saying this to Kylie earlier, so to her it just sounded like he was repeating himself. The Grundel can also be at times a sinister smooth talker.
This character was viewed by many alarmist parents as a possible child molester. While an over reaction, the Grundel's dialogue had lines that could be easily misinterpreted, such as "Come out and play child", "So fresh, so pure, but not for long", and "Open the window, we'll go away. Far from all the adults." 
- In his first appearance, the Grundel mutters something about "finding another" when he first manifests, suggesting that he was not a Grundel originally and instead was a child turned into one. However, he also may have been referring to a previous child he managed to manipulate.
- The Grundel was the only villain from the original Ghostbuster series to return in an actual episode of the Extreme Ghosbusters (though such villains as Samhain and Ghash did make cameos in the opening theme song and the Boogieman was mentioned in one episode).
- In a draft of "Grundelesque", the Grundel is known as "Grundel Astrala" in the Spirit Guide and the transformed Jack is classified as a Class 6 Astral. 
- The online multiplayer missions of Ghostbusters: The Video Game feature a Most Wanted Ghost also named The Grundel.
- On Page 10 of Ghostbusters Issue #4, Idulnas mentions Grendelspawn, a nod to the Grundel. 
- On Cover B of Ghostbusters Issue #12, the Grundel makes a non-canon cameo in the S.
- The Grundel makes a cameo on a special sketch cover of Ghostbusters #9
- On page six of Ghostbusters Volume 2 Issue #11, the Grundel makes a non-canon cameo on the page of Tobin's Spirit Guide.
- On the Convention Cover of Ghostbusters: Get Real Issue #1, the Grundel makes a non-canon cameo left of Samhain.
- The Grundel is mentioned in Kylie's bio in Crossing Over Virtual Trading Card #31, released on June 5, 2018. 
- Appears along with the second Grundel, Jack
- IDW Comics
- Insight Editions
- Tobin's Spirit Guide
- Section III: Metaspecters
- Pages 54-55
- Section III: Metaspecters
- Tobin's Spirit Guide
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 16:10-16:11). Time Life Entertainment. Egon says: "On my gosh! A Grundel!"
- ↑ Grundel (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 06:13-06:18). Time Life Entertainment. Grundel says: "And what you really want to do is invite me in."
- ↑ Grundel (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 04:35-04:50). Time Life Entertainment. Grundel says: "Gone. Oh yes, gone a long time. Find another. Oh yes. Another."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 16:19-16:28). Time Life Entertainment. Egon says: "A supernatural creature. A parasite. It appears to young kids. Tries to turn them bad. The more the kid falls under the influence of a Grundel, the more he looks like a Grundel until..."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 16:29-16:31). Time Life Entertainment. Ray says: "Until he becomes a Grundel."
- ↑ Egon Spengler (1997). Extreme Ghostbusters- Grundelesque (1997) (DVD ts. 05:57-06:05, 06:08-06:13, 06:20-06:23). Sony Pictures Home Entertainment. Egon says: "The Grundel. A supernatural parasite that preys on children by transforming the willing child into a Grundel like itself...a potential bad seed with the capacity, however hidden, for wrongdoing...The child must be receptive to the Grundel's lure of fun and games."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 19:06-19:13). Time Life Entertainment. Egon says: "You saw they boy, Peter. He's almost finished the transformation into a Grundel. The two are linked. If you hit the Grundel, you'll hurt Alec."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 19:21-19:24). Time Life Entertainment. Ray says: "Then all we have to do is get Alec to send the Grundel away again."
- ↑ Egon Spengler (1997). Extreme Ghostbusters- Grundelesque (1997) (DVD ts. 06:30-06:38). Sony Pictures Home Entertainment. Egon says: "That would be my first thought, Kylie but it's highly unlikely. You see, there's only one Grundel and Ray, Winston, Peter, and I trapped it 10 years ago."
- ↑ Grundel (1997). Extreme Ghostbusters- Grundelesque (1997) (DVD ts. 14:24-14:52). Sony Pictures Home Entertainment. Grundel says: "I knew Jack wasn't a keeper. Unlike the others, he was strong of mind and will. Your influence no doubt. So I placed him in a cocoon of sorts to let him mature... develop... gestate. Little did I know that Egon and his cronies were about to do me in. So there Jack remained in that tree until recent events."
- ↑ Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.55). Having won the child's confidence, the Grundel feeds on its victim's soul, replacing it with its own foul essence."
- ↑ Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.55). Paragraph reads: "We have found that the most efficient way to separate a Grundel (or any possessing entity) from its host is to douse the host with positively charged psychomagnotheric slime."
- ↑ Narrator (2016). Insight Editions- "Tobin's Spirit Guide" (2016) (Book p.54). CLASS III FULLY CORPOREAL PARASITIC BEING."
- ↑ Grundel (2009). The Real Ghostbusters - "The Grundel" (1987) (DVD ts. 05:03-05:04). Time Life Entertainment. Grundel says: "Need fresh. Untouched."
- ↑ Martin Olson scribd "Grundelesque" draft page 3, 6 11/22/13
- ↑ erikburnham Tweet 8/16/18
- ↑ TomWaltz Tweet 6/5/18