Maseki Gourmet: Mamono no Chikara wo Tabeta Ore wa Saikyou!

Thanks to the Goddess, he was reincarnated into nobility! That should have been great, but the skill he got, "Toxin Decomposition EX," was simply too plain, and he was made fun of everyday at home... However,one day, he noticed he could eat the magic stones of monsters with that skill and absorb their abilities. Furthermore, he was informed he was the prince of the neighboring country! What awaited him at the royal castle was an environment where he could train and experiment with the abundant amount of magic stones, bringing out his strong potential, and finally, even acquiring the ability of the legendary monster, the Dullahan! Surrounded by an assertive fiancée and a clumsy knight girl, the new life for the boy who will become a "King" begins!

(Source: Shousetsuka ni Narou, translated)