Spider's letter down

On the 10th anniversary of the Awakening, the day when everyone in The Pit woke up, The Spider, a known informant, criminal, assassin, and a gang leader invited 6 people, members of the gang, to his office. The only known trait between them was that every single one of them lived in The Depths for some period of time in their lives.
Spider ordered all of them to deliver a sealed letter to a certain spot, a route to which he described to them but ordered to not to reveal to anyone or they would be executed. The second order was to not try to open the envelope they were given under any circumstances.
Their journey would take them 10 days and would lead them down to The Depths layers. Should they succeed, they were promised a payment so big, they wouldn't have to worry about their needs for the rest of their lives.
And so their journey started.
On the 4th day, they reached The Depths.
On the 6th one, one of them had a very unwise idea. If Spider deals so much using various secret info, what if he learned what is inside the letter and sold it for an unimaginable sum.
He managed to take the letter without his coworkers noticing. The moment his fingers touched the seal, he fell dead. When the others found him, they discovered 6 stab wounds in his back. His hand was laying on the unopened letter from the Spider.
The rest of the journey was uneventful asides from the few moments where the party barely missed something large crawling through the darkness.
After 10 days, as predicted, they reached their goal. The place was some kind of observatory looking down one of many side shafts of The Pit.
An old man opened the door to the outside of his laboratory. He greeted the travelers. Once he received the letter, he got really serious. He stormed off into the laboratory and returned 20 minutes later.
He said to deliver the answer back to the Spider as soon as they can. His words went:
"No unusual movement below. Everything is in the norm right now."
The return took the other 10 days. The Spider invited them to his office to congratulate them. And while he assured them the payment will be given to them the same day, they could only focus on one thing inside his office. A pair of his daggers. They saw them before, most of the members of the gang did. But this time, one of them was stained with a dark red, dried-up blood.