Rexaar

This and many other bugs appear in Maple Story when your computer is on for more than 24 days (signed 32-bits integer problems) (how long your computer is on can be seen in the task manager performance tab)

Make sure you killed all 20 Terror Beast, the stage only goes up if all 20 terror beast are killed. Also you must not leave the map before the timer expires.
(use the terror beast quest as a guide to see if you missed any)

If you did indeed killed all 20 Terror Beast, then try doing a full shutdown by holding shift when you press the shutdown button in the start menu.
(Due to a bug involving signed/unsigned variables involving the time variable that MapleStory borrow from Window, MapleStory enter a glitchy state if your computer is on for more than 24 days (can be check in the task manger > performance tab > uptime), and only a full shutdown will reset it back to 0, normal shutdown/restart fails to do this properly due to hybrid boot)

The community did some research on this.
https://docs.google.com/spreadsheets/d/1od_hep5Y6x2ljfrh4M8zj5RwlpgYDRn5uTymx4iLPyw/pubhtml#

As far as I know, there are laws that require loot box rate to be disclosed, but potential lines aren't considered loot box because they're not actual items but rather improvement to an item, so there is no reason for Nexon to release the rate (yet, wiki say South Korea has plan to include improvement items but that was many months ago and nothing came out of it yet).

Does work on nodestone, but the effect is reduced (https://strategywiki.org/wiki/MapleStory/Formulas#Drop_Rate_Stacking say only half the effect is applied)

The sound is for this feature

To avoid overworking the game server, a wait time of 1 second will be added when selecting register, cancel, purchase, and receive at the Auction House.