Manga Death March Kara Hajimaru Isekai Kyousoukyoku

The story tells how a programmer gets into his favorite computer game, where he becomes one of the characters. At first, he cannot understand what is happening, he thinks that he is sleeping, but over time it dawns on him that his experiences are quite real. Thanks to his own abilities and knowledge, he managed to destroy a huge army of humanoid lizards, which eventually allowed him to raise a high level. He decides to retire, hiding his power, and live peacefully with peaceful people. However, the impending invasion of the demon king is likely to disrupt his plans.