Demlins were medium sized creatures with red-brown scaly skin and large blue and purple scales on their backs and upper arms. They had four blue eyes and sharp teeth. Their long arms ended with four sharp claws on each hand. They had long tails with striped black and white markings. They stood on two legs ending in feet with three sharp claws.
Demlins were aggressive creatures known to bite and scratch. They favored dark places like caves, and were known to attack in groups. Demlins generally stayed back in fights, taking turns to attack, then jumping up and slashing enemies with their claws.
During the High Republic Era some Demlins on Batuu were corrupted by an Ancient Sith Runestone, making them even more aggressive and ferocious than normal.
During the High Republic Era (circa 232 BBY to 233 BBY), Jedi Master Sylwin found an Ancient Sith Runestone and brought it back to the Jedi temple on Batuu to study with the help of Master Cohmac Vitus. The artefact caused the Jedi to turn against each other, leaving Mirialan Padawan Ady Sun'Zee as the only survivor. It also corrupted multiple Demlins in the surrounding area. These Demlins attacked Ady at the entrance to the temple where she fought off and killed many of them. Just as one final Demlin was about to attack, Jedi master Yoda appeared and threw the Demlin out of a window easily with The Force. Yoda then entered the temple through the main entrance, while Ady entered through the caverns. More Demlins had found their way into the temple and attacked Ady in the Main Hall, entering through the balcony doors. Ady and multiple fought them off, Ady destroying the entrances to where the Demlins were coming from and killed the remaining creatures. Ady later experienced visions of Demlins attacking her and Yoda until they managed to contain the power of the relic, pushing back the darkness together.
In the year 34 ABY, a droid repair technician encountered and fought many Demlins in the caverns in the Hissiq Springs while fighting the Guavian Death Gang, after being sent to retrieve two crates by the Utai droid technician Mubo. They then brought the Demlin scales back to Seezelslak's Cantina and gave them to the Azumel bartender Seezelslak along with acid-spitter tails and terateak pods for the Azumel to make a drink out of them. Seezelslak then told the technician the story of Ady Sun'Zee and the dark temple. The technician later encountered a Demlin in the caverns below the ancient Jedi temple while retrieving an ancient Sith relic for Ithorian collector Dok-Ondar.<ref name="Last Call">