The second part of the Alchemy of Souls OST is a song called 아리운 (Aching). While it is obvious to say that 아리운 means aching, I would like an explanation of what kind of word it is, as I can't seem to find anything similar in a dictionary.
My best guess would be that it is a combination of words like 그리움, 어려운, 아름다운... but it seems a little bit far fetched.
Here is the part of the lyrics where it appears and a link to the rest.
하얀 눈 위로 피어나는 꽃처럼
이 험한 길 위에 홀로 버텨온
아리운 내 마음
깊은 곳에 새겨본다