The Catastrophe was the name given to the event 12 years ago where suddenly two dimensions suddenly crashed and merged into each other, namely Earth and the fantasy world of Arcadia.  

As the dimensions collided, the Earth literally went back to the stone age as electricity, modern technology all went dark as 80% of the population were killed. Tidal waves destroyed the coastal cities and the surviving humans had to flee to the mountains. Earthquakes killed many and nuclear plants went into meltdown turning much of the land nearby into desolate wastelands.

Arcadia was ruined as well when the two dimensions collided killing over 3/4 of their population. However, Arcadia fared better than Earth due to the magic in the land, recovery was faster. Mages and magical artifacts were used to  clean their air, and their lands were not affected by the nuclear fallout. 

What was left was the merged continents of both Earth and Arcadia, with the Arcadian lands livable again after a while, but much of Earth a desolate wasteland. 

But as Earth was left with mere remnants of their people, with little to no technology, and limited fuel and ammunition, eventually the Earthlings just merged with the Arcadians in the Arcadian lands out of necessity. There was no longer Earth. The land was now only called Arcadia. 

As the lands calmed and populations merged, the humans found their place in this new Arcadia. They were refugees and asylum seekers. The Arcadians treated them just like other races, as the Arcadian lands had other humanoids like elves, dwarves, orcs and more. Besides, the humans from Earth were practically identical to the humans on Arcadia, except that they didn't know magic. 

Alison was such a survivor from Earth. The Catastrophe struck when he was 16 years old. That was the worst experiences of his life.

He joined the mercenary guild, as there was nothing else much he could do. As an Earthling, he didn't know magic, so he could only find work as a grunt who did exploring and miscellaneous jobs. 

A mercenary does quests, normally as an escort for travelers, but they mostly do exploring and treasure hunting. 

There were the dungeons of Arcadia, which pop up occasionally and have to be explored and closed, otherwise the monsters of the dungeon will spill out and cause death and destruction to the nearby Arcadian homes. The dungeons also contain treasures highly desired by both Earthlings and Arcadians alike. 

There were also tombs which appear. The strange thing is, the tombs are usually created by cultural relics of Earth, and if won, contain relics of incredible power. But they also appear to be the last remnants of Earth's culture. As such, the Earthlings treasure such relics, while the Arcadians just want them for their strange power. 

Lastly, there were the Earth ruins. There can be found ruins of Earth's cities and towns everywhere. Most of the stuff, like the food have already been ransacked. But once in a while, some remnants of Earth's civilization could be found and they are highly prized by collectors. 

Alison himself was by now a reasonably experienced mercenary and tomb raider. He was already a B class adventurer, after completing several dungeons and tombs, as well as souvenirs from the ruins of Las Vegas. 

--

A tomb had popped up in the ruins of Las Vegas was, so he decided to try the tomb together with his team, the Ironhearts. 

His team included the leader Aaron Irons a paladin, Rhonda Whitaker a mage, Thorsten Raller a knight, Bellarin Augustus a ranger and Sarah White a priestess. Alison himself was a thief. This team was a mix of Arcadians and Earthlings with Alison and Bellarin being from Earth. 

They weren't alone as there were other teams such the Deathbringers and the Ravagers, both much larger and more established than the rag tag team of the Ironhearts.   

The tomb was different from the normal tomb in that there were 3 entrances, each were where the entrance to one of the ruined casinos were. As there were 3 teams, they took one each.

Half a day later, Thorsten had already been severely injured and Bellarin had been killed in a trap. They faced one challenge after another, each time taking heavy damages. 

The tomb was filled with undead. From skeletons to zombies, wreaths to ghouls. 

Finally they entered a room and faced a deathknight in the center riding on a knightmare. 

Rhonda the mage immediately cast a anti-evil spell while Sarah cast holy magic. Aaron cast holy paladin magic on himself and charged with Alisan as Thorsten was too injured to fight. 

The death knight was immune to damage, no matter what spell or what attack they attempted. Alison attempted to backstab the monster as Aaron engaged it, but as he stabbed his duel daggers into the monster, it swung its arms at him and knocked him away. 

Alison lost his daggers as they were stuck in the shoulders of the creature in a gap in it's armor, but he grabbed onto something as he flew and crashed to the wall. 

Suddenly, much to everyone's surprise, the creature wailed and both the deathknight and the knightmare collapsed to the ground in a pile of bones. 

"What.. what did you do?" Rhonda asked

Alison stared at his hand, holding a necklace he had torn off the death knight. 

A voice boomed in his head. 

The Locket of Acererak, which grants eternal life and power beyond your imagination! 

"Is that the relic of this tomb?" Aaron asked, but Alison's eyes were still on the locket. 

Suddenly the tomb started shaking

"We've got to get out of here!" Sarah yelled as Aaron yanked Alison off the ground and they started fleeing. 

--

As they exited the tomb, the other teams were there and they looked at each other greedily. 

Suddenly both the Deathbringers and Ravagers looked at their small and injured team. 

"Run!" Aaron yelled as both teams started attacking them. 

Thorsen was immediately killed while the two girls Rhonda and Sarah were easily captured as they had no time to cast anything. 

Only Aaron and Alison managed to escape through the ruins of Las Vegas with many pursuers after them. 

They fought a furious battle in the ruins of a strip mall, with Aaron and Alison already injured. 

"Aargh" Alison screamed as a sword slash went across both eyes blinding him as he stumbled backwards, crashing into a old shop. 

He quickly got up and groped around in the darkness, trying to escape. 

Will You Use Me?

"Y...yes! If you can grant me eternal life, yes!" he said in his head as he head swords clashing behind him. 

Unlike what he expected, knowledge entered his mind. It didn't heal him and grant him immortality like he imagined, rather showed him a spell of how to create a phylactery and become a lich!

He stumbled blindly to get up and suddenly, he felt a sharp pain as a sword plunged through his back and out the front of his chest. 

"Where is the relic Ali?" a familiar voice whispered next to his ear. 

"Aaron? Bastard..." Alison said as he vomited blood. 

Realizing he had been betrayed, he started looking for something to create his phylactery to survive, even as a lich. 

He felt Aaron grabbing the locket from his hand and kick him off his sword as Alison's hands went flying, landing on a table filled with rubber objects. 

He stumbled and grabbed whatever his hands could reach and grabbed something that felt like a rubber rod and fell to the ground. 

"What is this? Why is it broken? You used it? Alison you bastard!" Aaron yelled looking at the locket, then at Alison. 

Alison lay on the ground, holding something strange and unfamiliar. He had no choice. This thing will be his phylactery as he cast the spell. 

"Is it done?" Alison thought just as Aaron stood over him and chopped his head off.