This story of mine starts right in your world, or at least one like it. Our protagonist being 16 years old on a Sunday afternoon was hanging around the living room, doing nothing in particular other than scanning social media. In all aspects he was just like any other teenager of that age if you disregard the really only negligible fact that he slept in a house of corpses. Which is a fact his social circle never failed to remind him of.
Victor Dunn was heir apparent to the multinational company that is the Dunn Funeral Parlour. You may wonder how it is that something such as a funeral company ever came to become a multinational behemoth - in the business of funeral services - and many shared your puzzlement. But it had happened. Theodore Dunn had started it in the 19th century, Hector Dunn was at the head of it currently, and Victor Dunn was supposed to inherit. Quite the curious ancestry, even I must admit.
It made for a curious childhood as well. However, Victor had long disappointed his father. He showed little interest in their ancestry and forfeited both the ownership of the family business as well as the craft itself. In fact, he vehemently rejected everything to do with funerals he wasn't necessarily confronted with during his childhood. Which means he knew little to nothing about the art of embalming or the process of cremating. And even the less obvious more subtle parts of an undertaker's duty like handling a family's grief and organising an appropriate gathering for the people left behind. So it should not be surprising that Victor also differed from what you might expect from a child of his particular background.
You might expect them to be socially inept, arrogant, and maybe to some extent somewhat shy. Victor was different. He was outgoing, popular in school and looked rather appealing as well. That does not mean his childhood had no effect on him. He was familiar with grief even with avoiding the business. He had no problem with corpses, seeing them quite simply as objects instead of a retired living being. And at this one moment, he was sitting on the couch thinking about the inevitability of death and how everything would eventually decay to nothingness, himself included.
[Not everything.]
A grey, holographic rectangle appeared in front of Victor. Words were written on it in white font. He sat up.
[This is the Death System. It's purpose is simple. The current ruler of the Land of the Dead, (?????), is in need of an agent who can perform all necessary duties in his place. Do you accept?]
"Come again?"
A second window appeared.
[This is the Death System. It's purpose is simple. The current ruler of the Land of the Dead, (?????), is in need of an agent who can perform all necessary duties in his place. Do you accept?]
He asked for it. Victor frowned.
"What do I get out of it?"
[Power. Eternal Life if you're good enough.]
"What do you mean 'good enough'?"
"I have standards", the voice was deep, frightening even.
Victor's head jerked up. In the corner of the room stood a large silhouette, clad in a long, dark robe. Its features were hidden by a cloud of black smoke.
"Who are you?!", Victor yelled.
"Shush, you don't want to alarm your father. I am that thing's creator", the figure gestured towards the rectangles still floating in the air, "You are try number 439,678 - I'm picky. If you succeed, you will gain a considerable amount of power. A god, basically. The drawback is simple. You will lead an immortal life, alone. Well, alone for some time. I will accept you resignation after, let's say, 500,000 years. You will keep your power. To sum up: You'll train for some time under the guidance of my system. If you win, you'll serve as my agent for 500,000 years. After that, you can do whatever you want. I'll even end your immortal life if you want me to. Trust me, that option will become attractive over next few millennia."
"You're not really selling it.", Victor remarked.
"Ah, bugger off. You don't believe me anyway. Few mortals ever rejected immortality when it was offered to them. Now answer me. Do you accept?", the figure sounded impatient.
Victor rubbed his forehead. He sunk back into the couch. After a long while he looked up at the figure in the corner. He nodded.
"Say it", the voice replied.
Victor hesitated.
"I accept."
Victor's scream echoed through the room. He felt a piercing pain in his chest. When it gradually died down, he noticed the figure had gone. Instead there was a new window.
[The system is now bound to you until your death.]
[To rule death one must experience it once. You will die in ten seconds:]
[10]
[9]
"WAIT!!", Victor paled and screamed. He was really quite fortunate there was quite some distance between his father's work area and the living room.
[...]
"Why do I have to die now?" Victor inquired in desperation.
[To wield your power]
"No, I sorta get why I have to die but why now?", maybe he could bargain.
[...]
[Efficiency]
"So there's really no reason.", Victor said relieved.
[Affirmative]
"So why can't you wait? Until I die? Naturally?", Victor carefully asked.
[I could]
"And will you?", he didn't manage to keep the eagerness out of his voice.
[Processing...]
[Processing...]
[No- wait, yes]
[You may continue with your life until you die. Afterwards, your training will begin. Do you consent?]
"I consent!", Victor answered full of joy.
[Acknowledged. See you later.]
All windows disappeared.
"Say what now?"
Victor was puzzled for a moment by that closing remark but quickly forgot about it. He was over the moon by apparently having gained a chance at becoming a god as well as narrowly escaping immediate death.
"This calls for some celebration."
Victor grabbed his jacket and keys. He quickly gave notice to his father and then stepped out of the house.
The next thing he knew, he was in a world of pain, rolling over the newly dented hood of a grey SUV.
After a short while Victor's breathing stopped.
Unseen by the mortals gathering around the site of the accident, a portal opened up above his corpse. It sucked in a hazy, transparent substance, then closed.
Victor had died.