Sources about Variphor
In a quick search, these were the sources that directly mention Variphor that I was able to find.
Texts on official website:
Text written on objects:
Sources that indirectly mention Variphor:
Based on these sources, who is Variphor?
By the update teaser and the texts on Yalahar's Stone Carving, we get to know "Variphor" is the name the Yalahari gave to a powerful entity. By interacting with it, they began to fear it. They also believe this same entity is responsible for many dire and destructive things which happened in the past. The teaser also states the true Yalahari who returned to Yalahar to bring destruction to the world during In Service of Yalahar Quest (Azerus) is influenced by Variphor.
It seems Prince Drazzak is at least a supporter of this very same entity which was named Variphor by the Yalahari. This can lead to theories of Variphor being related to Roshamuul's history of destruction and the appearance of demonic creatures.
The NPCs Cerebrir and Dedoras also know the entity by its Yalaharian name. They state Variphor is the one after The Godbreaker and the knowledge to use it. This makes Variphor the one behind The Secret Library Quest and either the one behind Ferumbras in Ferumbras' Ascension Quest or someone with the same objectives as Ferumbras, i.e to obtain The Godbreaker.
What the NPC Messenger of Heaven refer as "the thing from beyond", "the thing" and "this thing" seems to be Variphor. According to the Messenger, it was the Yalahari who gave the thing a name, this made it stronger and eventually caused the Yalahari to vanish from the world. This also explains why the Messenger doesn't refer to it by "Variphor" or other name. It is also believed that the thing is behind the World Devourer and Heart of Destruction Quest.