Precisely what sort of realm Dwersuun was before the Great War struck may never be known - it was destroyed more thoroughly than even the wastelands of Eleghost or the surfaces of Irallith or Stygia. For a time it remained a barren land, then Maris the Mad (so called because he was mad, even for a spark) discovered it and decided to use it as his personal treasure storage. One physics-defying device later, and he attempted to summon the lost treasures of whatever culture(s) had inhabited Dwersuun before its destruction.
He succeeded (if not in the manner he'd intended).
He did not survive his success. A great temple, complete with accompanying temple grounds, materialized above his head and fell to the ground. His device, however, did survive, and continued to churn away underneath the transplanted temple. Over time, other lost temples and religious centers (including some only recognizable as such by experts which might no longer even exist) began to stud the empty lands. Plant life from countless worlds began to spread across the earth, creating forests and lands seen nowhere else. Wildlife also came along for the ride at times, and spread along with the plant life.
While not as dangerous as the wastes of Eleghost, Dwersuun is still very dangerous albeit (usually) in subtler ways. Some temples are harmless, most of which have been looted already, but others were dedicated to forces still present in their grounds and yet others to obscure technologies many of which still function. As well, within the ground of a temple time may not perfectly match up with the time of the rest of the realm, though the mismatch is usually not very severe. Some adventurers who have made it back alive report seeing figments, ghosts, within the buildings and transplanted lands, perhaps memories of the previous inhabitants or perhaps merely other time-lost travelers.
Dwersuun is a legendary location amongst those who travel the realms - many have heard tales of it, but few have actual proof of its existence. Getting there is reputed to be difficult indeed, but courtesy of Maris' still-functioning invention it's nevertheless easier than getting back out. Some have speculated that those who have been there before ought to be able to return there easier, but given how few have been there and escaped and how few of those are willing to even consider going back again...