There is a place in the Sevni Kingdom called Niag. It is a land of mountains, frozen with the bitterness of ice and the suffocation of snow. Within these mountains are two large rivers that come from one of the peaks.
The Magna River and Glacei River are very real, and dragons can see where the river empties into the ocean. Most, however, dare not to follow the rivers to the peaks and find their source.
It is said that where the two rivers join, there is an ice cave. That cave is the entrance to Taliig, underworld of dragons. Once a dragon crosses into the cave, any water touched within will suck the soul out of the body, and trap it there until it is judged.
For this reason, only few have dared to search for this cave. None have returned.
Further into the cave, you will supposedly discover it increase tremendously in size. It appears to be an entirely new world. The joined rivers become Morem River. It is known to hold the countless souls of dragons until they are judged by the Dragon God of Death, Henviras.
If Henviras deems you fit, your soul can swim to the surface, and crawl out of the water into the paradise known as Coelum. This side of the Morem River, there is plenty of plant life and beautiful sculptures. You can find your family there and live in peace and joy forever.
Of course, if you are not deemed fit, you will be doomed to Tiignu. This side of the Morem River, there is nothing but barren wasteland. Mortal dragons argue that Tiignu is ice cold, while others claim it is hot like the angry fires of the Sun.
In Tiignu, you will forever be hungry with no way to eat, dehydrated with no way to drink, and tired with no way to rest. It is eternal torment.

So when a mortal dragon dies, Henviras is present. He saw your suffering and dared to touch a paw to your heart. Once he touches you, you are separated from your physical body. Henviras then flies you to the cave in Niag, and convinces you to go for a swim in the waters.
The rivers act like a giant funnel. They bring all the souls he's collected to the massive Morem River to await judgement.

While most think Henviras is the cruelest of all the Dragon Gods, he is actually the kindest. Without him, dragons could not die. Those who are suffering will never be put out of their misery. Dragons would never be able to find their families in Coelum, and balance would be lost.