There was once a man who was summoned to another world of fantasy and magic. After a while of surviving, gaining skills and even moving up in the adventurers guild, he died. Many years later he was reborn...

  As a dungeon core.

  This is his story. (now I can tag this as a reincarnation, summoned to another world, dungeon core story!)

  BTW he lost a bunch of his memories.

  Over many decades, a forest far to the north of the great city of Argrovad had been growing in its mana density. So much so, that monsters were spawning and breeding there at an alarming pace. Because of this, the area has been devoid of travelers for a long time. Only the bravest of adventurers dare to get anywhere close.

  When the mana density reached a peak, the forest shuddered for miles.

  A large amount of the mana started to get sucked towards the center of the forest. As the mana was getting pulled to the center, it was so thick, that it looked like a giant circle in the shape of a blue wave crashing in on itself.

  When the mana was gathered at a single point, it coalesced into an ocean blue, fist sized crystal.

  All the monsters in the area started to panic Some ran in all directions, some started viciously fighting amongst themselves.

  One curious boar headed to the center of the condensing mana to see what happened.

  "OH MY! WHAT THE HELL IS HAPPENING???" (this is the dude, not the boar)

  The curious boar began to sniff at the unprotected dungeon core.

  "I was reborn as a dungeon core?"

  Before he could answer, the nearby boar got too curious, and gobbled up the dungeon core, it was too hard or its teeth to penetrate, so the boar swallowed it whole.

  Giving into his fate, the un-named, summoned dead guy made a choice.

  "Yes I would like to claim this area as my dungeon"

  (that was the system panicking)

  "Ummm, yes?"

  After waiting for three whole minutes, there was a response.

  The boar dropped dead on the spot like a sack of bricks.

  The un-named, summoned dead guy, had a slight intuition of how things worked.

  "Status"

  This story has been taken without authorization. Report any sightings.

  "options"

  "upgrades"

  The dungeon core had taken place right next to the boars heart.

  He could feel every inch of the boar.

  "I can't just lie here like a sitting duck."

  "I don't have the underground protection like other dungeons, so first I gotta deal with protection."

  "I spend one point on mobility"

  First he started looking around, his limbs were jerking slightly.

  "I thought I would be able to tell the boar where to go, I did not think I would be the freakin boar!"

  He stood up quickly, and jumped and ran around a bit, to get used to the new body.

  "Looks like I'm a half a meter tall male boar."

  "I wonder what dungeon expand does"

  "WAIT I DIDDN'T SELECT THAT!"

  His boar body dropped to the floor, he began to squeal in pain.

  (oh by the way up until now all his words were in his head)

  "SQUEEEEAL"

  "SQUEEEEEEEEEEEEEEEEEEEEEEEEAL"

  After an excruciating minute and a half, the pain stopped, and he stood back up.

  Checking out his body, he had grown a lot, now he was a meter tall. (with all four feet on the ground)

  "whoa that was a serious upgrade."

  "oh that’s good."

  "How do I get more mana?"

  "What happens to the extra mana when I'm full? can I store it away?"

  "I see it's at 15 now, how long will it take to increase?"

  "What is my dungeon cores mana?"

  "why cant I see it in my stat screen?"

  "But I can just ask you anyways so whatevs yo"

  "What can I do with my dungeon core mana?"

  He was feeling hungry and began to eat some grass.

  "Cool"

  "uhh, grow grass!"

  "Whoa, I feel like I have to take a huge dump"

  "Wheres that grass I just made?"

  "whoa gross!"

  "well I guess its not that gross"

  "I mean I have had worse things in there"

  "Well not in this boar body, but you know what I mean"

  "OK well lets try to poop out some grass!"

  He squatted a bit, but try as he might, nothing was coming out.

  "Hey whats going on? why cant I poop?"

  "oh"

  "well lets try moving it with my dungeon core mana"

  "I select dungeon core mana and move the grass to the outside of my body"

  "Why didn't that work?"

  "OK, OK"

  "focuuuus"

  With his mind, he easily plucked the small patch of grass off of the wall of his large intestine, and worked his way through the winding maze until it came out of his backside, he then replanted it on the top of his back.

  "Whoa that was easier than I thought it would be"

  "whats that smell?"

  "Oh gross!"

  He focused again, and was able to remove the grass, he tried planting it on the ground but could not, so he just left it on the floor.

  "If eating grass lets me make grass, I could eat other stuff too!"

  He started to eat random things that wee lying around.

  "Lets see what I got here, Sun Lillies.... Rocks, I could be a rock mage or something... DEER POOP? WHAT THE HELL!"

  *spitting

  *spitting

  *licking a tree to take away the deer poop residue

  "Gotta be more careful next time."

  Just then, he heard something moving in the bushes some distance away.

  He wearily looked to see what it was.

  "oh hey its a deer!"

  "Hi deer, I'm a boar!"

  The deer looked at him suspiciously, then it hissed and pulled back its lips to show long sharp bloody fangs.

  "Hisssss"

  "Oh what the crap is that!"

  The un-named, summoned dead guy now living as a dungeon core that possessed a boar turned around and started running as fast as it (he?) could.

  Unknowingly in the direction of the nearest human settlement. (even though it's really far away, but he is really scared and kept on finding scary monsters to run away from until he got kinda close to the human settlement, after stopping for some breaks, and a long run on sentence)