First of all "Mystic Treasure" is a quest that can be taken at each town’s Metaphysician starting at lvl 30 map (Skysong). You have to pay a certain amount of gold to take this quest.
The quest will start where you have to kill a certain amount of monsters. After that, submit your finished quest back to the Metaphysician, you will gain a decent amount of exp, and will acquire an item that can be used to activate opening chest quests. (Dig for treasures!)

In this guide I will show you the chest locations and the possible spawn spots of Deathmark Gilhook.
You will know if you spawned a Deathmark Gilhook because it will be announced serverwide. Deathmark Gilhook’s are rather weak, but gives a big amount of exp and lots of loots.

This guide is only 90% accurate, cause I can’t pinpoint coordinates of the Deathmark Gilhook’s spawn spot, and because they may spawn anywhere around the map, you may have to move around different possible spawn spots to kill it, and be reminded that you’re not the only one who will be hunting for it. So, you should really run and kill it as fast as possible. lol

Skysong:

The Billows:

Doom Bog:

I’m sorry If my English is not perfect.

More maps will be added as soon as I level. lol

Use this guide well.

Credits go to Raine.