Well, as a brief answer: No, the Planegazers and The Last Planegazer are not creatures from another dimension.
As for the Planestrider, well it isn’t really a creature, it’s more of a concept, which is much more complex to explain briefly, but I’ll try to summarize what I know regarding the plot, its lore, and some few details:
PLANEGAZERS
The Planegazers were a secret society of humans formed to the sole purpose of answer one simple question:
“What lies behind everything?”
As that question suggests, they were devoted to the pursuit of knowledge, or more specifically, the true origins of the universe itself (beyond what is known through Genesis, by the way). Through intense research, this secret society managed to gain access to important places, characters, and artifacts, which would eventually shine a faint light on what truly “lies behind everything.”
Among the most relevant events of their research is the discovery of a mysterious chest (in Ab’Dendriel) by Dolis, the Headmaster of the Planegazers. Ignorant of the chest’s origin, meaning, and purpose, the society decided to investigate its mysteries, which led them to discover encoded messages in objects around the world, known as 'The Five Walls of Testimony' describing five powerful entities of Tibia: Zathroth, Qjell, Garsharak, Ferumbras, and Zoralurk.
*It’s worth mentioning that these messages were encoded through binary numbers in objects, which must be discovered gradually as you progress through the quest.
During these events, the Planegazers came to understand that an 'entity' (or rather, a concept) called the Planestrider is the true force responsible for these messages and for the entire plot surrounding the mysterious chest. Although at first they were not sure what the messages meant, they suspected the Planestrider left them intentionally to them.
Thus, the Planegazers decided to build 'forges' at the 'sites' where the Planestrider had been present throughout Tibia’s history (lorewise). In these forges, they conducted research on discoveries linked to the Planestrider and its influence on those places. For example, in Oramond’s forge, replicas of the Ab’dendriel stones were built in order to try to decipher the code embedded in the objects.
*this forge serves as a “hint” to realize that the Menhir in the Ab’Dendriel temple also has a code engraved on its surface.
There were also various events that occurred during their research, such as the construction of the opticording sphere, which functions as a kind of artifact used by the Planegazers to record and store the information they found (which is why, during the quest, you basically do the same thing the Planegazers once did in the past using that object).
Another important event is when Dolis (and perhaps more members of the Planegazers) found and possessed an incomplete cube. Even though they didn't know its purpose or use either, they at least suspected it could only be used once it was complete.
Finally, during these investigations they decided to create a place called 'The Chamber of Knowledge' (Gnome Deep Hub), where they would gather their findings and the specimens found at the sites they investigated.
After this... the Planegazers simply disappeared.
..
The rest of the story, in terms of conclusions and derived stories, could be left to personal interpretation by each player. Yet there are vague details and scattered hints that eventually explain what happened after. For example, one of the most basic and well-known hints is the encounter between Galthen (who possessed the missing part of the cube) and a Planegazer in Carlin. However, Galthen never shared that missing piece (at least his own) with any Planegazer, nor with anyone. For that reason, you find the missing part of the cube in his tomb.
PLANESTRIDER
As for the Planestrider, it's hard to go too deep into that topic for one reason:
Its explanation could partly depend a lot on each player’s interpretation, and therefore a 100% definitive answer would be somewhat irresponsible. But even speculating a bit, to truly understand its purpose in detail, the player would need very advanced prior knowledge of Tibia’s lore, specifically from Yalahar onward. But as many probably already suspect at this point, there is information that could lead you to conclude its 'identity' -or at least,- its 'resemblance' to Variphor... but even that fact, could only be the tip of the iceberg.
What is certain is that this concept (Planestrider) is not and cannot (yet...) physically appear in our dimension as such, but it can communicate through certain means... one of them being the dimension that contains the cube: the dream dimension (which is why you can use the cube as a place to sleep if you use it in your home).
Now, the Planestrider does not actually have a defined physical form. And even how you see it (from an RPG perspective) is how you would imagine it (in theory), but it doesn't represent the 'absolute reality'. Still, you can at least notice that its representation is made out of 'improvised materials' (metals, conduits, etc.) in order to acquire a physical form that you can understand. Because by its nature (as I said before), it doesn't have a defined form in the physical plane.
Deep down, 'it'... is 'that': a concept.
It’s like trying to give a physical appearance (for example) to the word 'happiness.' In that similar case, you are the one who can shape that concept into a specific form in your mind, but it does not represent an “absolute truth”.
..
Tibia in the last 10~ years (in terms of lore) has been heavily conceived through metaphysical concepts. However, this does not mean the content itself has become abstract, interpretative, or lacking in foundation/information within the game. In fact, quite the opposite, there are details and hidden secrets within the game that will naturally guide you to understand each of these concepts, and eventually, they will also shine you a faint light on 'what truly lies behind everything.'
...
*I’ll leave here a timeline I created about the Planegazers some time ago, in case it helps anyone interested.
Even so, keep in mind it may contain wrong concepts or interpretations, just like this whole answer might as well... by the way.
The rest I leave to your speculation and points of view.
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Main Sources:
-All books about Planegazers in all the forges around Tibia.
-Galthen Book in Bounac: https://tibia.fandom.com/wiki/Personal_diary_of_Galthen,_The_Found_Man_(Book)
