Sum up (made by basing myself on the novel):

As planned, MC & his group evacuated their little fort, leaving behind a nice present for the next occupants. Some harpies also stayed behind in order to check the situation and to trigger the explosion.

When the enemys army arrived, they are surprised to find a fort there in such a harsh environment, and even more to find it empty. I do believe that, in the novel, MC had placed a warning sign, telling them basically to f*** off if they dont want to die. Well... In any case, this fort is great for them! As said previously, this is a dangerous place and having a fort there might serve as a bridgehead for their future attack against the elves territories! Who would be able to stop them n-... Oh. Right. The explosives. *BOOM*

MC & group are trying to see the fireworks when they suddenly happens. Too bad that it was just a short show! ...But you can imagine the power of the explosion if they were able to hear it from so far. After that, MC checks his titles / achievements. The Nice boat one is Lady-killer that give him +10% ATK against females (this might also boost his attractiveness as well!). The condition to unlock it was getting the favor of at least 20+ women. So... Yeah, quite a lot of women wants to sleep with him.

The other titles are: First Kill (need to kill one human - +5% ATK vs humans), Tough Guy (lvl 20 - +50% PhyAbilities), Assassin (100+ kills without being detected - unlock takedowns), Bomber (100+ being killed with explosives - +10% DMG when using explosives), Mass Murderer (1000+ people killed at once! You did it! - +10% ATK vs humans) and Champion (3000+ kills alone - increase abilities of allies within a 100m radius by 10% and increase favorability).

So... Yeah as we can say MC uses Explosion. Its super effective. The harpies come back and confirm it: The whole army was basically wiped out. There were some survivors (less than 1000 out of 5500) but they are now in a very hostile territory... and the gizmas are quite hungry. So it miiiiiiiight have been better for them to die during the explosion. Only a few are able to escape from this little hell.

Considering that MC just killed quite a lot of people, Isla and Sylphy are concerned about his mental health (MC says that he doesn feel anything: He was away from it and didn see it). Isla proposes to spoil MC when he gets back. Sylphy is jealous and proposes to do the same and MC happily accepts. However he also do the same things to her, spoiling her. Its very effective against Sylphy who is still considered as a child by elf standards.

Oh. And on the next day, of course, our little Sylphy is quite embarrassed by what happened. Her being spoiled, her losing herself and everything else that happened. That and the teasing trigger the second explosion of this chapter! Run away!!