The Red Eclipse.

But it is understandable that the realm tree will rely on the immortals since the major aim of the realm trees is to create strong and powerful origin gods. The trial of heaven is done to ensure that. 

While the era of conquest is bringing mass culling of the population of the realm tree, such a thing isn't new. It has been happening for quite a while. The will of the realm culls the weaklings of its population every origin cycle through the trial of heaven and produces immortal soldiers with the resources available. 

Of everyone who attends the trial of heaven, only 20% of them survive. The remaining 80% all die and hundreds of origin gods are produced each origin cycle. So people have always been dying. The rule of survival of the fittest has always been upheld. The era of conquest has only ramped things up. The increase in deaths doesn't affect origin gods much so it is allowed to happen.