The Eldren were a race of beings that lived thousands of years ago, prior to the arrival of humans on the main continent. Not much is known about them, besides them being the creators of Elderglass. What happened to them is a mystery.
The Bondsmages of Karthain claim to keep their operations in small scale, and choose to build out of stone and mundane materials, in an attempt to not befall the same fate as the Eldren, stating that while they do not know what happened to them, they do know that the Eldren created magnificent, showy, special structures that stretched higher and outshone anything else before or since, and possibly drew attention to themselves from some other power.
There is an example of Eldren language (possibly, though no one is totally sure of the source) at the Echo Hole in Camorr. The walls of the Elderglass cube are carved with an unknown language no one has been able to decipher.