God the setting in this world is so fucking metal. The world tree being slowly corrupted by the demon lord's severed spine goes super hard.
The comparisons to WH40K are obvious but what I really love is how creepy they've made the reincarnation process. Most isekai handwave the issue away by having the original consciousness "die" due to illness or the like before the requisite black haired Japanese teen take it over.
But here we have Nui who genuinely loved the original princess and slowly comes to realize that she's not really here anymore, and is at risk of completely disappearing. And yet the samsara user is not really at fault for this situation either, he was grafted onto its host body by a goddess with no intention of hurting the princess.