[Forgotten name] I can’t remember the name of this Manwha that I started reading but lost

Hi there, I saw a video about this Manwha where the main protagonist is a wizard hero called Leon who defeats the demon lord Belfast and goes back to the kingdom. He is summoned by the king and beside him is prince shark who tells the hero that there is a cave of monsters in the neighbouring kingdom and that many people have gone in and not returned. Leon goes to the cave accompanied by prince shark and ventures in only to be stabbed on the back by prince shark with a sword coated in a paralysis poison and tells him he was in the way of prince shark marrying the princess.

Does anyone know the name of this manwha?