In this series, upon reaching the age of 16(?) each person goes through a rebirth ceremony. The normal people it assigns them a rank based on what a persons maximum potential is.
The MC is a void, instead of assigning him his maximum, it gives him a ranking of 1. The regular average human is somewhere in the teens.
Voids are unknown at the start of the series, but our MC levels fast and discovers the path to advancement at certain thresholds comes by absorbing the powers from other voids you kill.
The absorption does not just happen, its a battle of wills between the killer and the killed. And if the killed wins this battle of wills they take control of the body of the killer and the original host just dies.
So book 4, starts just after our hero has closed a dungeon called Warrior Titan, he was killed by the boss of the dungeon, another void. This was a case where the dead won the battle of wills and he now possesses the body of the boss. The problem with this is two fold, not human, and it had tortured his fiance in the dungeon.
The fiance we find out if pregnant with twins, with one of the twins drawing strength from her, as its a void like the child's father. Voids when they are born always kill their mothers.
So now our MC on top of the usual level up and get stronger, is out to save his fiance, from their unborn child.
There are other plot lines in this book like saving his brother from a dungeon he was thrown into because our MC failed to close Warrior Titan, which as you see above he did do, just someone assumed and got ahead of themselves. Another line, was making a pack with other voids to work together, a totally unheard of path for voids.
Over all, it moved the story along, was not all fight fight fight, and am not wondering if ill read the next one.