Omustardo | Doomsayer Completionism and an Odd Bug @Omustardo | Uploaded October 2012 | Updated October 2024, 19 hours ago.
The Doomsayer allows you to toggle certain warning messages after you've experienced them a few times. He's right next to the lumby home teleport. Since the ability to toggle a warning is a permanent unlockable, it is completionist content by definition even if it isn't required to wear a comp cape.
runescape.wikia.com/wiki/Doomsayer
runescape.wikia.com/wiki/Mort_Myre_Swamp#Mysterious_teleporting_squares