Five stone tablets can be found throughout the Underground Pass dungeon. They play a role in the Underground Pass quest, giving a similar sense of danger to the messages that occasionally appear in the chatbox when walking through the dungeon, before killing Iban in the quest.
The stone tablets appear to praise Iban or attempt to scare people going through the dungeon, which ties into the theme of the quest that people will lose their mind when venturing too deep into the dungeon. They also claim Iban is the son of Zamorak, and provide a slight bit of information on quest progression.
Reading the first stone tablet.