The Demon Realm's Eternal Dragon is summoned in the 18th episode of DBD. The episode starts with Goku transforming into Super Saiyan 3 to continue fighting Gomah (who has been powered up by the Evil 3rd Eye/Tertian Oculus). Vegeta, Piccolo, Shin & Glorio land near Bulma, Dende, Panzy & Neva to rest after attempting to fight Gomah. Panzy barely recognizes Goku after he transforms into Super Saiyan 3 while Shin urges Goku to give it his all against Gomah.
Gomah doesn't seem too concerned by Super Saiyan 3 as he writes it off along with Goku's other abilities as "silly magic tricks" and asks how many more Goku has available. However, Gomah learns too late that he underestimated Super Saiyan 3 as Goku manages to land a solid hit on Gomah and still puts up quite a fight against Gomah afterwards. Bulma, Panzy, Kadan, Kadan's present soldiers and even the "Majin Brothers" cheer Goku on as he continues to fight Gomah. Noticing Goku has powered up gives Kuu an idea and he heads off somewhere to put this idea into motion.
It doesn't take long before Duu & Tamagami 1 join Goku in fighting against Gomah. Eventually, Gomah is able to knock the trio down and they end up unconscious. Kuu returns to Gomah's arena with a giant chocolate chip cookie to feed Duu which succeeds in helping Duu regain consciousness and Kuu encourages Duu to power up. Goku manages to regain consciousness and proceeds to power up which inspires Duu on how to go about the advice Kuu gave him; seeing Goku's Super Saiyan 3 Form also inspires Duu to change the back of his head to resemble the spiky hair of the Super Saiyan transformations (much to the confusion of Arinsu).
Goku & Duu proceed to charge towards Gomah and Tamagami 1 regains consciousness to help continue the fight. The trio are able to put up a fight against Gomah…but Duu eventually tires out (which Kuu takes to mean "the cookie wore off") which gives Gomah a chance to punch Duu elsewhere. Goku & Tamagami 1 continue to fight Gomah until Gomah eventually uses the Evil 3rd Eye/Tertian Oculus to blast them with an energy beam that knocks them to the ground. Gomah beats up Goku a bit more to leave him unable to stand before he begins to charge an energy ball to finish off everyone else within the arena.
As Gomah charges his energy ball, Neva approaches Goku and aims a ball of magic towards Goku. When the ball of magic makes contact with Goku…it triggers a transformation that results in Goku transforming into Super Saiyan 4, the transformation that was created for GT. The form does have a slight redesign compared to its design in GT with one of the most notable differences being that the arms become bigger (resembling gorilla arms); another notable difference compared to the GT design is that Goku now has red eyes & red hair (unlike the yellow eyes & black hair of the GT design) which bare a strong resemblance to the Super Saiyan God Form. It's also worth noting that Goku is still a kid when he transformed here while he became an adult when he transformed in GT; as far as I know, an "in-universe" explanation was never given for why that happened in GT but the most popular fan speculation I've seen range from "Super Saiyan 4 brings the user to their physical prime" to "the Super Saiyan 4 transformation is more powerful than the Black Star Dragonballs (the set of Dragonballs that turned Goku into a kid in GT)".
All the characters currently in Gomah's arena are surprised by the transformation Neva triggered within Goku with Gomah vocalizing his surprise before launching his energy ball. Thanks to the power of Super Saiyan 4, Goku is able to block the energy ball to send it elsewhere afterwards while Panzy notes how different Goku looks in Super Saiyan 4 Form and Piccolo notes how Goku's energy signature is "surging" right now. Goku now has a moment to actually see how he looks in Super Saiyan 4 Form while Neva smiles with a nod and Gomah growls in anger. After taking in his new transformation, Goku prepares to continue fighting Gomah.
Goku & Gomah charge towards each other to continue to fight each other. Thanks to the power of Super Saiyan 4, Goku is able to put up an even better fight against Gomah and even incorporates some monkey like movements while fighting. Goku's display of power helps give Piccolo, Shin, Glorio, Duu & Tamagami 1 the resolve to resume fighting against Gomah. The 6 fighters are able to work together to fight against Gomah who doesn't have as much of an advantage as he did earlier on in the fight.
During the fight, Piccolo launches the Special Beam Cannon alongside Glorio using his purple lightning while Gomah tries to counter with his own energy blast which leads to a beam struggle. Gomah ends up winning the beam struggle but it provides a distraction to give Goku a chance to charge a Kamehameha. Gomah manages to punch Goku while he's charging the attack…only for Goku to still succeed in fully charging and launching the Kamehameha towards Gomah. This results in a crater being created where Gomah was hit by the Kamehameha.
Unfortunately, the power of the Evil 3rd Eye/Tertian Oculus allows Gomah to recover…and transform into an even bigger giant form. Gomah manages to overpower Goku and uses the power of the Evil 3rd Eye/Tertian Oculus to trap Goku in an energy sphere which forces Goku back to Super Saiyan 3 Form and then forces him back into Super Saiyan 2 Form. Bulma, Panzy & Glorio are all worried about Goku…but Glorio is interrupted by a call from Arinsu who instructs him to bring the Demon Realm's 2-Star Dragonball & the Demon Realm's 3-Star Dragonball to her. The energy sphere Goku is trapped in forces him back into Super Saiyan (1) Form and then forces him back into his Normal Form before disappearing.
Glorio manages to retrieve the Demon Realm's 2-Star Dragonball & the Demon Realm's 3-Star Dragonball from Hybis by simply offering to watch them because "it's getting too dangerous" before he reaches the area where Arinsu & Kuu have placed the Demon Realm's 1-Star Dragonball. Arinsu confirms to Glorio that she is ready to make her wish to the Demon Realm's Eternal Dragon to obtain the position of Supreme Demon King while boasting about how having her wish granted will allow her to defeat Gomah. Arinsu also admits that she instructed Glorio to learn the Namekian language so that he would be the one to summon the Demon Realm's Eternal Dragon (who she calls "Porunga" which is also the name of Namek's Eternal Dragon) and make the wish on her behalf. After thinking back on the time he spent with Goku & the others (while Arinsu urges him to hurry up), Glorio eventually speaks the Namekian language to summon the Demon Realm's Eternal Dragon which catches the attention of everyone currently within Gomah's arena.
The Demon Realm's Eternal Dragon resembles Porunga except with a red color scheme and being even bigger in size; supplementary material refers to the Demon Realm's Eternal Dragon as "Demon Realm Porunga" for the sake of distinguishing him from Namek's Eternal Dragon. This leads to Goku's group realizing that Glorio had been working for Arinsu which leads to Vegeta reacting in anger while Panzy reacts with sadness; Neva notices how Glorio had learned the Namekian language. Like other Eternal Dragons, Demon Realm Porunga (voiced by Chris Sabat) gives a speech explaining to the one who summoned him about he will grant a wish that is within his power; Gomah attempts to attack Demon Realm Porunga to prevent a wish from being granted…only for Demon Realm Porunga to easily block the attack and flick Gomah elsewhere. When Glorio hesitates, Arinsu cheerfully reminds Glorio to word the wish along the lines of giving her the position of Supreme Demon King "with unparalleled magic power" but when he still hesitates she growls at him to do it; Glorio proceeds to speak the Namekian language. The episode ends with Demon Realm Porunga granting Glorio's wish.
Naturally, the biggest contribution of this episode is the fact that DBD has granted Super Saiyan 4 (GT's biggest contribution to the franchise) the coveted "canon immigrant" status; I've seen some refer to this moment as "Akira Toriyama's final gift" and I can see why some refer to it as such. I'm one of those fans with the sentiments that GT had some good ideas/concepts but not the best execution so while I may not have been a fan of the way GT executed some of their concepts, I still think some of these concepts have great potential and would be open to seeing certain concepts be given the "canon immigrant" status; Super Saiyan 4 is one of the ideas/concepts that I feel has great potential and I was pleasantly surprised to see that we had the chance to see Akira Toriyama's interpretation of the transformation. I give GT a lot of credit for the build up to Super Saiyan 4 and while I can understand why some felt the transformation came out of left field here, I feel Neva using his magic to unlock the form for Goku could be seen as a "preview" until Goku can use the form at will much like how the Tournament of Power could be seen as a "preview" until Goku can use (Autonomous) Ultra Instinct at will. While I do like the yellow eyes & black hair of the GT design, I also feel that the red eyes & red hair of this design gives the form a look of "uniformity" that compliments the red fur; it's also worth noting that GT already set the precedent of Super Saiyan 4 having red hair via Gogeta in the 60th episode of GT. Likewise, it's also worth noting that GT already set the precedent of one being able to transform into Super Saiyan 4 without a tail as Vegeta was able to transform into Super Saiyan 4 in the 59th episode thanks to being showered in Blutz Waves from a machine Bulma built; once the Blutz Waves "wore off" in the 62nd episode of GT, the tail Vegeta regrew from the transformation disappeared. While I can see why some think the bigger arms look ridiculous, I can appreciate the way how they go along with the "monkey look" of the form (in addition to the monkey like movements shown during the fight with Gomah) and I get the feeling that the bigger arms would look less ridiculous on an adult compared to a kid. Sean Schemmel mentioned in a recent interview that he collaborated with Stephanie Nadolny to come up with a "kid version" of the voice he gives Goku while in Super Saiyan 4 Form for consistency which is a level of dedication I both admire & appreciate. This episode also has another GT reference in the form of Demon Realm Porunga whose status as a red colored version of a preexisting Eternal Dragon that is also even bigger & more powerful than the green colored version acts as a nod to Black Star Shenron (the Black Star Dragonballs' Eternal Dragon); considering the fact that DBD is set 3 years before DBS, it seems safe to say Glorio isn't going to go through with the wish Arinsu wants him to make which would give Glorio some parallels with Giru from GT.
