Belial (I am the Final Boss)

Belial was sent to look for the recently reincarnated demon king. Since she was previously engaged with the demon king, she looks forward to being reunited. Unfortunately she's attacked by multiple players looking to get xp and loot and after dispatching some weaker players a higher level player shows up and gives her some trouble. After a scuffle, Belial sees the MC who's hiding during the battle and recognizes him as the demon king. Before she can approach her love, her attaker uses the distraction to split her in half with a flying slash attack. To put salt on the wound, the MC doesn't remember he's the demon king and loots Belial's body.