Before we get into the details, I’d like to shed some light on who the Ruthless Seven are and who belongs to this group.
The demon race is divided into several groups. One of these groups are the Archdemons, and the Ruthless Seven are demons of this very type. They serve Zathroth and have fought battles against the Nightmare Knights, among others.
Apocalypse
The Total Destruction. CipSoft members summoned him on Test Servers (May 2006, November 2009 and July 2012).
According to Tibia Fandom it is one of only two monsters in all of Tibia that can use Armageddon spell. In the Ferumbras Ascension Quest we meet his minion - Razzagorn.
Fun fact: Apocalypse was defeated by players on a Test Server but didn't drop any loot.
Ashfalor
He WAS the General of the Undead. We learn more about him from the book The Dreamers V.
During the battle that took place on Roshamuul, he was torn to pieces by one of the demon princes — the Prince of Agony, Sephorath.
His minion, Ragiaz, like the other emissaries of the Ruthless Seven, appears during the Ferumbras Ascension Quest and is accompanied by Death Dragons.
At this point, part of the answer becomes clear — Ashfalor is dead. We probably won’t encounter him in the game (though we can’t be certain of anything).
Bazir
Known as the Great Deceiver or Lord of the Lies.
In his throne room, located in the Pits of Inferno, you’ll encounter many traps and misleading puzzles.
You can read about him in The Dreamers IV and The Dreamers V, where he also employs his tricks.
His servant Zamulosh uses similar tricks, becoming invisible and teleporting.
Infernatil
Infernatil, the Incendiary of Hell like Apocalypse, was summoned on a Test Server in 2012.
From the book The Dreamers V, we learn only that he is one of the “more furious” demons. His throne room, like his servant Mazoran, embodies Hell itself.
Pumin
We know he's the Lord of Despair, which I'm inclined to believe, because in his throne room, A Dead Bureaucrats give off exactly that kind of vibe. Not to mention the presence of Spectres and Betrayed Wraiths.
In the Ferumbras Ascension Quest, he is represented by Tarbaz
Tafariel
She's the Mistress of the Damned, and judging by the creatures in her throne room (Lost Souls, Banshees, Dark Torturers), that would make sense. She's the only female in the Seven.
Shulgrax is under her command.
Verminor
Finally, we have the Plaguelord, master of Plagirath. Summoned on a Test Server in 2012.
Some people say he might have been involved in the creation of vampires.
Now that we have everyone here, I’ll explain where demons come from.
Demons do not naturally exist in our realm. From the book Archdemons, we learn that they enter our dimension through a kind of gateway, which can only be opened if certain conditions are met.
To summon a demon, you must gather enough synergistic energy to first open a rift.
One of the components is negative energy, which resembles the essence found in the demonic realm. During this process, demonic energy seeps into our world, amplifying the power of the other ingredients. As a result, the rift grows larger, allowing increasingly powerful demons to enter our world.
This ritual can be initiated from the demon side and from our side. Morgaroth, Orshabaal, and Ghazbaran are excellent examples of this.
In Morgaroth's case, his summoning is preceded by a volcanic eruption on Goroma 
This is likely the main factor that makes it possible to summon such a powerful demon.
For both Morgaroth and Orshabaal, the ritual of summoning takes place in our realm, as the messages indicate:
Morgaroth
Orshabaal
But when it comes to Ghazbaran, the situation is quite different 
There is no indication that anyone is trying to summon him, yet demons are beginning to enter our realm. This amplifies the negative energy, widening the rift, and eventually demon master himself appears:
Ghazbaran
It is safe to assume that the ritual was performed on the other side (the demon's realm).
All right, but where are the Ruthless Seven?!
Demons cannot remain in our realm indefinitely. Upon arrival, strong demons (e.g. Orshabaal) feed on negative energy to stay here longer, but this energy also sustains the rift to their realm. We read about this in the book Archdemons. It also contains information about the demon Shervad the Destroyer, who wore armor made of an unknown material that allowed him to remain in our realm for at least three centuries. In his final battle, he lost his strength, even though he was not physically injured.
From another book, titled Demonic Invasions, we also learn about a gateway connecting our realm to the demons’ realm, though it is currently inactive. It explains that it works much like a volcano: sometimes increased activity is observed, but usually nothing happens, and from time to time there is a massive eruption, and that is when the demons are most active.
The problem is that these eruptions aren’t stable enough to allow for a longer campaign without the risk of many powerful demons getting stuck in our world “like fish on dry land”. This explains why demon leaders rarely appeared on the battlefield - they were most likely summoned by their minions for a time.
Putting all this information together, we can draw some conclusions:
Since the books say nothing about the deaths of any members of the Seven, with the exception of Ashfalor, I assume they are alive.
As they are unable to remain in our realm for very long, it is highly likely that they are in their own realm - the realm of demons.
The Dream Master says that they (the Nightmare Knights) managed to seal away the evil forces of the Pits of Inferno, at least for a while, and that it was obvious they couldn’t hold them off forever.
I think that in that case, a future encounter with Ruthless Seven (though now it’s more like Six) is quite likely (CIPSOFT, PLEASE!).
I really like the old Tibia Lore; it would be great if, instead of constantly getting new content, we could get some of the old stuff revamped.
I hope that my answer gets you to like Lore too, and that it helps you understand Ruthless Seven and demons in general a little better!
