A rock;

As soon as Kerren appeared in the dungeon he started running in a random direction. It didn't matter where he will end up, the only importan thing is how far it was from the entrance.

He needed as much freedom as possible to "experiment" on the mosquitoe.

He didn't even know how he would fight it or even see, thus he needed time to figure it out... as soon as possible.

15 or so minutes later he arrived in a place that he felt was nice. It was a little meadow somewhere in the forest.

After he used some spells to scout the area to make sure that not a single person was nearby he he let out his "aura" to attract mosquitoes.

Immediately after he did so - they appeared out of nowhere! This time, however, Kerren coulds sense them with his spells! It was due to the modification he made based on two skills Yusaya gave him last time!

It was far from perfect, however it did allow him to sense those mosquitoes! As it turned out it was exactly the same case as with the entrance to the secret room!

It didn't matter how powerful his spells were, unless they had a part of a special skill inside of them they wont ever see what's hidden!

Kerren suspected such a thing after the last encounter, but to think that he was right! It pleased him greatly as it was one of the rare cases where he didn't need to change his plan a lot!

Since it worked he started doing what he initially planned. It wasn't that big of a deal now that he knew how to decect them, but killing them was a big deal, as it later turned out.

It didn't matter what he tried... He couldn't damage them! As if they didn't exist at all!

He tried many methods, simple physical attacks, complex magic spells he spent lots of time making and even simple skills that he had!

Sadly, none of it worked! Not even skills that he thought were the solution... It could only mean that there's some kind of method he wasn't aware of!

It could be anything, from mental attacks Kerren didn't know at all to some random buff that allowed him to attack the boss.

The latter was completely normal since there were lots of dungeons with such a mechanic implemented in them. They waried from dungeon to dungeon but in general it was all the same.

One neede to acquire a certain artefact or a "permission" to kill the boss, it could be anything, a sword, pendant or even simple prayer in a correct place.

Kerren, however, wasn't sure about that, since in the book he read about the dungeon there wasn't just a thing.

However, judging by the fact that the guy was nothing but a pig who liked to spend money it was entirely possible that Yusaya found the solution on her own.

As such, he didn't have any options left. Even if found the main body of that mosquitoe... so what? He couldn't hit his clones or whatever those were!

Yes, it was possible that one didn't need any method to kill the mosquitoe itself, but it was required to kill it's cloner, but so what?

If it was the opposite Kerren could even die, since those had an ability to suck mana out of it's target! One could say that it was his arch-nemesis!

As such, he needed a method to deal with any technique that boss had!

Thus, he decided to look around the dungeon. Now that his spell could locate the mosquitoe, it meant that it could also locate an entrance to some secret room that'll hold the answer!

However, it still needs to be tested... That's why he started running towards the location where the previous entrance was!

It's not like he remembered the exact place, he knew the general direction.

...

After an unknown amount of time, that wasn't long at all, he arrived to the place. He didn't find it at first but after some time he finally located it.

It was a bit harder than locating the mosquitoe as it seemed that someone placed several barriers around it, preventing people from even approaching.

They couldn't stop Kerren, which was obvious. As he approached the the branch he started studying it. It's behavior, what changes in his spell allowed his to see the "secret" part clearly etc.

It took him several hours before he finally made a "final" version of his spell that'll help him to see entrances to secret rooms in dungeons!

More than that, he suspected that it'll work even in other dungeons, since they were built by the same principle!

As such, he now had another way to get rich! Of course, it didn't mean that he'll go around and look for them, but if he enters a dungeon that there's a secret room there? Dont mind going in then!

After some more thinking Kerren decided to revisit the secret room, but he couldn't even enter it. It meant that those were one-time use!

Thus, he needed to be as fast as he could be to find more room than Yusaya! He didn't know how many of those she already found, so he immediately activated his spells to it's maximum!

It did waste lots of his mana, but it was worth it! He found another strange object that looked like an entrance! It was a normal looking rock 2 kilometers away from him!

Less than 2 minutes later he was right beside it. He didn't enter it as he wanted to test his spell some more. He then changed it accordingly, to see if there were any differences with the branch. As it turned out - there were none.

As such, it was time to enter that secret room! Kerren touched the rock without any hesitation and disappeared immediately!

When he opened his "eyes" he found himself in a room that at a first glance looked similiar to the previous one, he, however, wasn't even thinking about the room right now!

Another epiphany invaded Kerren's mind! His eyes! Why could he even close them, if he didn't have them? If he did, yeah, closing them would mean that he wont be able to see, but... Eyes on his body didn't even work in the first place!

As such, he removed his eyes entirely and... He could still see!.. I have to note that this process wasn't painful at all since it was done with a single thought, even if he added scratches or "wounds" on his body he didn't feel it, so dont worry, he's completely fine even without eyes!

It made him think a lot and he was almost sure that he wasn't human anymore, but so what? He had more important matters to think about! Now, at least...

He started inspecting the room, to his surprise it looked the same as the other one! Well, at least as he remembered it...