I just created an article for Samael in the English TibiaWiki with a summary of my findings about him:
Samael managed to switch worlds during a small ritual performed by Eclesius to help his former assistant, Turian, who is now transformed into a frog. While Samael was bewildered right after entering our world, Eclesius managed to lock him up in a cage located under Eclesius' home. Keeping Samael's cage locked and in good shape is needed to "prevent the apocalypse", according to Eclesius who refers to him as a Demon Uberoverlord.
Its name is confirmed by his fellow inmate, A Confused Frog, who describes Samael as an annoyed Demon Overlord.
It is not possible to see Samael from any known place in Tibia, however it is possible to hear his screams from outside his cellar, from [a] cave in Thais sewers and from Thais prison.
— https://tibia.fandom.com/wiki/Samael
Everyone acquainted to Eclesius' tasks know he says he has locked a demon in the basement. Although, this demon was never seen on the game window and here's why: it is located after extra bars inside his locked cellar (to the east of the "screenshot" below). So, even if there is ever a bug that allows players to open any closed door, it wouldn't be possible to fight Samael.
The fact that there couldn't be found any info about Samael in fansites (and even on Google searches) for so many years is that his name can only be confirmed by saying "demon" to the NPC A Confused Frog while on the correct part of the The Scatterbrained Sorcerer Quest. Also, Samael rarely screams, and his screams could only be "heard" from directly outside his cellar, from two specific map fields in the Battle Axe Quest - which is accessed by pick - and from the underground of Thais Prison.
How I Discovered It
Back in 2015, I managed to get an up-to-date list of all creatures in the game. Samael - with capital S and no article, typical of boss monsters - was one of the only names in such list that I had never heard of before. Then, I tried to cast the spells Summon Creature and Creature Illusion on it and I got the message "Sorry, not possible." which confirmed a creature with such name existed, but unfortunately I couldn't even see how it looks like which is still a mystery.
Years later, on the Test Server for the Summer Update 2020, I checked Team Finder's lists of Bosses, Hunting Places and Quests. The boss list reminded me that Samael was still in the game and still to be found. During the TS, I reported 6 other bosses from that list which were either unreachable or unkillable. After they were removed from the list and the update was launched, I questioned the devs if Samael should stay on the list and, as usual, they gave a vague answer and decided to keep it there. I don't know if their decision would have been different if I had included Samael in my original report, however, in such case, I would have to act as I were sure that Samael can't be killed which I wasn't.
During the Test Server for the Vocation Adjustments, in last October, I can't recall why, but I decided to visit the somewhat recently-expanded underground of Thais Prison and I was surprised by this scream:
I immediately realized it could be a scream of the never-seen Samael, then I started my investigation. Even before making the A Confused Frog NPC talk about him, I tried to locate the origin of the screams. As you can see, the scream on the screenshot below is centralized, indicating Samael is located straight to the south from this position. When standing one field to the west, the screams were slightly to the right. Curiously, these two map fields are the only places in this cave were someone can hear Samael's screams which indicates he is located exactly 30 fields to the south.
When I was making this test, I noticed it was a lot rarer to "hear" a scream from this place which made me believe Samael could be walking north and south. When he is on the south, he would screen 31 map fields away from me and I wouldn't be able to hear him. Therefore, I made more tests, and was able to get two screenshots of the same scream, one slightly to the north and the other slightly to the south, which confirms Samael is able to walk north and south as I suspected.
While doing The Scatterbrained Sorcerer Quest, I asked all NPCs about demon and Samael and I was able to get the following replies which confirmed the never-seen screaming creature is indeed the mysterious Samael:
During Mission 6:
Hunter of Dragoes [225]: Demon
A Confused Frog: That demon is called ribbit. Erm, no, I mean Samael. He is one REALLY annoyed demon overlord, but we get along okay. Not sure if I'd let him out of there though. Ribbit.
Hunter of Dragoes [225]: Samael
A Confused Frog: Yep, he told me one day that that was his name. In the tone of voice that makes your ears almost explode and singes your hair.
After Completing the Quest:
Hunter of Dragoes [225]: Samael (or Demon)
A Frog: I've grown quite fond of the old chap. Or then again, no I haven't, really. All that anger can get on your nerves. Ribbit.
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