The note turns out to be a trap by Amber.
Backstory of Haunt, Morrain and Amber: Haunt (Male Tiefling), Morraine (Female Shifter) and Amber (Male Warforged) were amateur thieves and old friends. They took a contract to steal some artifacts from an Assassin Guild. The Thieves Guild that gave them the contract sent their second in command (and Son to the Guild Master) ‘Noble’ the ironically named Tiefling.
After sneaking into the Assassins headquarters, and locating the weapon artifacts, Noble double crossed the three rogues and attacked them. Amber lost and eye and fell out of the 2nd story window, Morraine and Haunt fought off Noble, but he escaped with a Frost Rapier Relic ‘Winter Shard’. Haunt and Morraine found Amber outside near death. They stabilized him but were forced to leave him as the authorities had arrived to investigate. Haunt and Morraine went their separate ways, keeping on the run.
10 years later: Haunt is still on the move, until he meets up with Tarrasol, Alistair, Moloch and Thraxus to form an adventuring party. He has heard rumours of a Warforged with an Amber Gemstone on his chest that is operating with an assassins guild, he has also heard that Morraine is living somewhere on the Sword Coast of Toril. Noble’s father has died and he has become the new master of the thieves guild of Calimport. END BACKSTORY
The letter Haunt received from Morraine was a fake. Amber sent it as part of a trick to use Haunt to find Morraine. Once Haunt had followed Morraines trail to her hideout in the hills outside Waterdeep, Amber struck. He was now part of the Assassins guild that they had tried to Rob. He, along with 6 other Assassins, attacked the adventurers while shouting accusations at them. During his outbursts he mentioned the Assassins guilds new allegiance with Noble’s Thieves Guild and how Noble had told everyone that Haunt had escaped with the Frost Rapier ‘Winter Shard’.
Haunt and Morraine with the help of the rest of the party convinced Amber that Noble had taken the rapier. He accepted their apology for leaving him and offered to find out more information on Noble from within the Assassins Guild. He said the Assassins Guild would likely continue to hunt them until he had proven Noble’s Guilt. From their they again went their seperate ways, Morraine said she would keep on the move near the sword coast and that she would keep an eye out for them, Amber said he would contact them in Luskan when he had more information on Noble. Haunt left with his companions on the Sea Sprite for a sea voyage north toward Luskan and The Spine of the World.