If you're looking for manga similar to The Saga of Tanya the Evil (Light Novel), you might like these titles.
For twelve years, the virtual world of Yggdrasil has served as the playground and battlefield for the skeletal lord Momonga and his guild of fellow monsters, Ainz Ooal Gown. But the guild's glory days are over, and the game is shutting down permanently. When Momonga logs in one last time just to be there when the servers go dark, something happens--and suddenly, fantasy is reality. A rogues' gallery of fanatically devoted NPCs is ready to obey his every order, but the world Momonga now inhabits is not the one he remembers. The game may be over, but the epic tale of Ainz Ooal Gown is only beginning...
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Both series are well written and have a fairly gritty setting. Both also see their characters descend into a sort of madness, while for Ainz it is due to most of his emotions being suppressed by the system, for Tanya it is due to both her attempt at furfiling what is required of her while simultaneously trying to rebel against Being X.
they are very similar in concept and tone. Just try them both and I'm sure you'll see what I mean.
Kurata Tsubasa is not your average citizen. Sure, he’s a genius programmer and software engineer, but as far as he’s concerned, his most important trait is that he’s a hardcore mecha nerd. So, what happens when you take that nerd and reincarnate him into a fantastic world of swords and sorcery? Well, you’d think he’d be disappointed...but this world has magical giant robots! Obviously, Tsubasa—reborn as Ernesti Echevalier—now wants to devote his life entirely to these wondrous pieces of technology. But how exactly can young Ernie achieve his ultimate goal of getting his very own mech? He’ll have to rely on his imagination, uninhibited by the common sense of this new world, as well as his extreme dedication and focus! He also might just find help in the form of a rather interesting quirk...
In the Great Orient, lies the continent called Rodenius. One day, an unidentified flying object arrived at Qua-Toyne Principality’s airspace. The unidentified flying object defied the Principality’s common sense. But that unidentified flying object was just a harbinger of something even more shocking. Far to the eastern sea, a group of islands suddenly appeared. The country of the islands called itself: Japan. According to them, their nation had been transported to another world. Now finding themselves separated from their old world and must survive in the new one, how will Japan interact with the native countries of this world? Will they greet her as a friend or as an enemy? This is a story of the country called Japan as it was engulfed in the chaotic storm of the world.
On a certain summer day in the year 20XX, in broad daylight a gate to another world appears in Ginza, Tokyo. An army and a swarm of monsters come storming through the gate. The Japan Ground Self-Defense Force is able to defeat the invaders and they head through the gate into uncharted lands. An epic fantasy about a battle between the human race and another world filled with dangerous enemies and beasts!
Herian was the ultimate player of the nation-building VR game “Tactics Chronicle” when he got transported into the country he built himself in the game. There, he met the great army of magical beast which possessed their own ego, as well as his peerless former NPC generals. Hiding his identity, what he found together with his vassals were the Elves battling against a great evil. Seeing the tears of the innocent masses, he came to a realization of his duties as king and made a declaration in anger. To vanquish evil with the hundred thousand magical troops under his command! “Stand up, my brave people! Now is the time to bare your anger!” ““OOOOOOOOOOHH!!!” The moment the thunderous roars resounded—the peerless Alchimaira breaks off ahead!! The king of a thousand beasts now orders the attack of the strongest army!!