I vaguely remember the logo for the manhwa being a skull on fire in a phone.
And the main plot is that the MC was going to become the demon lord, then a female knight who came from the future was supposed to kill him, but saved him instead. And some story plot happens and she died protecting him, and now he is just going around waiting for her to be born so they can be reunited.
The first chapter starts off with a girl using an app that transported her into another world where she can "level up" so to speak.