I changed the time the story is written in to simple past. Also: from now on 'this' is the MCs thoughts.

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The third room was the largest, twice as large as the other two combined. Inside there were many tables with glass objects in different shapes on them, although some of them were toppled to the side. ‘An alchemists’ laboratory? It would explain the neighbouring rooms.’ In a corner of the room there was a strange corpse. It had decayed to the point where only bones remained, but the clothes it was wearing still seem new and didn’t fit that image. The large patch of dried blood showed that this person had indeed died there, but it couldn’t have been too long ago. According to the information I got about undead these signs meant that this was one of the weakest undead types: the skeleton. The flesh had most likely been turned into energy for the newly created undead so the necromancer wouldn’t have to invest as much himself.

 Luckily it was just lying there which gave me the perfect opportunity. I prepared my hammers, rushed close and struck its skull, crushing it to pieces. The skeleton twitched but went limp shortly after the head was destroyed. Then I noticed something: On one of the skull fragments there was a tiny crystal, almost too small to notice. ‘Could I maybe absorb it? And what would I gain by doing so?’ While pondering about these questions I heard something from the side.

I turned around and saw that another skeleton had risen from behind one of the fallen tables where I couldn’t see it. I slowly approached the new threat, being prepared to retreat at a moments notice. The skeleton tried to hit me with a knife it was holding in its right hand, but I dodged to the left and manage to strike one of its legs, causing it to lose balance. As it fell to the ground it stretched its arm again to land a hit, but I simply moved out of its reach and smash the outstretched arm between my hammers. Then I got close to it again and crushed it’s skull. 

‘Phew. That went surprisingly well. I guess a single skeleton doesn’t pose much of a threat to me.’ After a quick sweep of the room, to make sure it was safe, I investigated the remains. After some searching I found another crystal on a piece of the second skeletons skull. I brought it close to the cube and it was indeed absorbed. It seems these crystals are what operates the skeleton and contains its memories. ‘According to my knowledge weaker undead aren’t really sentient beings. They just follow orders and react to what happens around them according to their directive. And it seems that all of this is managed by the crystals on their skulls. But that also means that they will fall apart if the area around the crystal gets destroyed. This information will certainly be useful’

You could be reading stolen content. Head to the original site for the genuine story.

 I absorbed the other crystal and searched through the rest of the room. But I found nothing I could use so I just returned to my hideout for now, since I wanted to try something. After closing the door I went to the stone tablet and touched it. After a short while it showed the number 313. ‘Yes! I can really increase my strength by absorbing crystals. I should probably try to hunt as many undead as possible before the person controlling them notices my existence.’ 

With new determination I leave my room and walk down the corridor in the opposite direction of the rooms I already searched. It ended in a staircase which lead upwards. Climbing stairs wasn’t easy, but nothing I couldn’t do. At the staircases top was another corridor with many other ways forking off and another staircase at the end. Over the next two hours (‘For some reason I am capable of judging how much time passed. I simply know how long it was’) I searched through all rooms on this floor. 

Most of them were used to store things. I found lage amounts of food and stuff like lamp oil. But almost all of it had become unusable. Someone seemed to have put put a lot of effort into destroying all supplies in the castle. It wasn’t hard to figure out who. I also encountered some skeletons, 23 to be exact. Some of them were lying on the floor and I could defeat them with a surprise attack. Others were walking the corridors and I had to fight them, but there weren’t any serious problems since they only attacked me one at a time. ‘I will probably get dangerous when I move further up.’ After returning to my hideout I checked my strength. The stone tablet shows the number 327. ‘Seems like I will be able to use a new arm soon.’ Content with the progress made I returned to exploring the castle.