I was unable to find an exact door that this key opened. But the first Daemon (Demon) was originally found in Mintwallin so if the description on the key is accurate I assume that is where the key was used.
"The first Demon spawned near present day Mintwallin, south of the current city, and was a lot weaker than today. It disappeared in a 1999 or 2000 update. It was named Daemon and looked a bit different."
"One player run ahead of all others to be the first at the demon to see what treasures he would hide. This was Galadriiell, the probably mightiest sorcerer back then. She wouldn’t have had to hurry that much, because there was no one else that was so eager to see the big red one, but still she did. There was only one place in Tibia back then where you could encounter a demon, and that was deep below Thais, south of Mintwallin."
Sources:
https://tibia.fandom.com/wiki/Demon
https://tibia.fandom.com/wiki/Golden_Helmet
https://drive.google.com/drive/u/0/folders/1i8ncdzgi6c3lc6FbnpePh9cNBFZqDWku