I am looking for a fantasy manga where a boy was killed and reincarnated to a fantasy world where he does not have a status of a knight, but his power is strong against poison. He make a star crystal out of a poison rose for his mother and archduke granddaughter. He return with his mother to his father's land and suddenly becomes crown prince.
In this world, It's only me who can get stronger by eating Magic Stones!
At one point, I received an offer to have a smooth life reincarnation from the Goddess-sama... But the skill I got as a prize is Toxins Decomposition EX. Ugh, what a plain skill! Its so pitiful that even my own family makes fun of me...
However, one day I realized that because of this skills effect, I can eat and gain power from Magic Stones! I've also been notified that I'm the prince from a neighboring country!? What awaits me in the castle are days of experimenting with Magic Stones and training, the perfect environment for me to become the strongest man there is...
And to top it off, I received the Magic Stone of Dullahan!
The story of a boy on the path to become a King while being chased by an assertive fiancee and Female Knights starts here!
Found with a search for
isekai manga poison archduke which led me to this Reddit post.