Walk around mileth Tavern until you catch the beggar's attention.
He has a horrific tale to tell if you are willing to spare him some gold. Spare him 1,000 coins, enough to live a month.
After the short story select "What do you mean by terror of sleep"?
Then "I'm ready, and do have two committed fellows here"
"I think I can take it, with my fellow's help."
and "I will. I am. Here are my companions."
He will then ask for the name of the second group member.
They will receive a popup where they must click "I am, and so are my companions." before typing the third group member's name.
If you are doing this quest as a duo they can type your name and you may begin. If you are doing this as a trio then the second group member must enter the name of the third group member.
Once the pact is complete he will tell you the location of his nightmare within Mileth Crypt.
Head to Mileth Village (93,54) and enter the Crypt. From floor 1-1 head to the north corner (5,5) and down the stars to floor 2-1.
As you begin to head south towards (40,40) you will notice something the beggar had mentioned in his dreams and will be summoned inside the Shrouded Crypt. Slay the creature from the Beggar's nightmares and leave the Shrouded Crypt.
Upon leaving you will be rewarded with the Legend Mark: "Slew Terror of the Crypt (#)" and some Experience.
The Experience varies depending on level:
Level 4?-9 = 50,000 Experience
Level 10-19 = 200,000 Experience
Level 20-29 = 550,000 Experience
Level 30-39 = 750,000 Experience
Level 40-49 = 1,000,000 Experience
You may repeat this quest every 12 hours.
Note: You can only slay the terror of the alley, the garden OR the crypt.
Completing one means you won't be able to start the others.