Sute
Class: Any
Level: 28
Location: Eldritch sanctum (Buya mage's guild)
Items in inventory: None
Sacrifices: 200 coins
Rewards: Sute's key or 2500 coins, legend mark

1) Go to the guildmaster of the Buya mage's guild.
2) Ask him about Sute.
3) If you want to skip the story, just ask what must be done.  Otherwise, let him tell it.
4) Accept the powder.  You'll turn a funny multi-color.
5) Go to the cave east of the north gate in Buya.  The powder will disappear as you enter.
6) Explore the cave until you find Sute's Nest.
7) Slay Sute, and take his key.
8) If you wish, go to Forsaken Grotto and use the key in front of the treasure chest.  You will be rewarded with 2500 coins.  If not, you can just keep the key.  It can be used any time later also.
9) Return to the mage guildmaster.  You'll get your legend mark, and will be able to get the powder again and re-enter the cave at will (limit once per hour or so).