Monster no Niku wo Kutteitara Oui ni Tsuita Ken

"Eat Or Die!" The first prince of the kingdom of Farune, Mars, is only twelve years old, yet he lives in fear of assassination. In order to avoid the deadly poison in nearly all of his meals, he slips out of the castle into the nearby monster-infested forest, where, battling monsters and his own hunger, he has to hunt for his meals in order to stave off starvation. Then, one night, as he's searching for his next bite of monster meat, a red-haired woman shouldering a greatsword appears before him and says to him, "You have promise. Become my apprentice." How is he supposed to tell her that this must be some sort of mistake? But there's a secret that only she knows, and that he's stumbled upon: eating monsters makes you stronger! With her training and his newfound power, Mars will finally overcome all those pesky assassination attempts. However, that doesn't end things; no, it's only the beginning of a nationwide—even continent-spanning—series of wars and plots with him right in the middle!

(Source: Cross Infinite World)