Although this cave theme plays in underground dungeons in seasons 1, 2 and 4, it is first heard in season 1, when the party (then with only 5 members) sneaks through a cave that goes under the river separating Dascillia and Celestion. Originally, it had a lot more organic drums and orchestral instruments, but I forewent those ultimately, deciding on a sort of New Jack Swing meets 90s JRPG sort of feel.