This review is gonna be for both the manga and the novel since they don't have much of a difference in terms of story progression. There may be some veeeery subtle differences but doesn't affect the whole flow. The main difference I can say is just the art.
***Skip to the last for TL:DR***
Moving on to the main review, it's a damn great read.
***PLOT***
The plot and even the comedy is great. Also, our dear MC is one with almost no talent and thus after accidentally obtaining an immortal body, he studied as hard as he could just to get back at those who scorned him for his lack of talent. But after he learned those things and decided to show it off to those who mocked him, they were long gone. Fast forward to the present time of the story, he had spent at least 5000 years studying the arcane arts.
The story starts out with the MC, Asley, and his familiar, Pochi, inside their dwelling, a renovated dungeon, and after some time and persuasion from the latter, they decide to go out on a journey, leaving their home. They meet a bunch of people from a ruined and abandoned town, help them get up on their feet, and even teach a few kids the arcane ways. In order to learn more of the world, he sets out on another journey to enroll in the Magic University in Beilannea together with his familiar and one of his students. After which, they go to school, learn even more things, and participate in battles, and go adventuring. Pretty linear plot so far.
Further down the line, due to circumstances, he was forced to leave Beilannea, and after his return, he had to set forth for another journey. This is where the plot gets exciting (at least for me). Due to God's influence waning, the absence of Holy Warriors became a huge concern for the world as they are the only ones who can defeat the to-be-resurrected Devil King. His next journey is set for the past where he was transported there along with Pochi to fulfill the missions that were given to him. After which, they return back to their original time line and prepare for the impending invasion and awakening of the Devil King.
I think that's a shortened, dummed-down summary so far.
Thinking about it again, the plot seems linear. The arc wherein he gets transported back builds the foundation of the history that is known in his era. Although he thinks through things, like hesitating whether he should interfere in current events which may lead into a discrepancy in his era, they were all actually part of the 'plan' which God had already predicted and foreseen. At that point, he realised that the actions he takes, more or less, wouldn't affect the timeline he came from, save a few times he realised that some dots needs to be connected like asking a favour from one of the Heavenly Beasts, the Violet Phoenix Shi'shichou in the past actually had some connection to the future.
Also, as his returned is set in stone, on matter how long he stays in the past, he spends quite a long time in antiquity. And while he does so, the state of the Nation where the other characters are become quite chaotic. To the masses, it's as if nothing changes but in actuality, they are being led by the nose of Ishtar the Black, who is assumed to be a devil controlling the upper echelons of the nation. Asley and Pochi in antiquity builds strong bonds and connections that will soon serve him well in the future when he returns. As he makes connections and forge bonds, the bonds he had forged in the future also does their own thing to survive al the while keeping in mind the actions Asley had done for them.
The plot line is quite interesting (although I've pretty much summarised the events until volume 09.... sorry :v), albeit the forced comedy from time to time. But there are also some times where the mood becomes serious when it needs to, not that it does every time.
Other elements from the whole story also emphasises the point where devils and demons are evil and humans and God are justice. It's refreshing since most plot lines nowadays are either reversed where devils and demons are justice and those who revere God and the church are evil. If not that, then they are neutral. Point of interest as well lies with the elves, being labelled as dark elves as they introduced the devils into the world.
***CHARACTERS***
Moving on to characters. There are a fuck ton of them, but rest assured, you will be able to remember their names, or at least just their looks. Starting from half of volume 04 and volume 05 onwards of the novel (WN), there will be a lot of characters that will be shoved to your face. But at least their names and probably faces and features will be repeated over the course of the story. They would have links to different important events of characters. Some would even prevail till the future era. And among those who are introduced, no one is as significant as Asley and Pochi. Not because they are the main characters but mainly because of their interactions with each other and with others. Besides them are stereotypes such as a kind-hearted lass, cool yet carefree womaniser of a hero, condescending antagonists, muscle-for-brains type, you name it and it's most probably there. Besides those, Asley and Pochi are just labelled as eccentrics.
***ART***
As I've written down earlier, the art from the novel and manga are worlds apart. From book cover to content, there is a clear difference especially when it comes to details. Both are great in my opinion but if I were to choose, then I'd prefer the art of the manga. Since the manga is still quite a bit away, as compared to the novels (both LN and WN in JP), which are already completed, it's worth to wait for the release.
***TL:DR***
Art, great. Plot, good. Characters, hmmm perhaps.
Both the novel and manga have great art, the plot is somewhat linear and is easy to follow, but character development is somewhat lacking in my opinion as they were replaced with comedy (and forced comedy). But with characters, besides the development, there is also the entanglement and bonds. Almost every character introduced will be somehow connected or related one way or another unlike some stories where after their introduction and appearance in a single volume, they die down and never to be brought back up again until the end of the series.
To further add to the TL:DR plot, even when including space-time continuum stuff, it acted just as a tool and maintained a linear progression. Like a fait accompli of sorts wherein his actions, no matter what he thought and done, are already set in stone and would not affect the space-time continuum. Get me? Well even I can't explain it unless the novel was read ¯\_(ツ)_/¯ .
All in all, it's a great read with intertwining events and characters, refreshing plot (read the last paragraph of ***PLOT***), eccentric main characters but at least a comedic duo, and also great art.
