This is an easy quest that can be completed by speaking to Naylie, a dead woman in Hallowtown. She's not in one of the houses, but is walking around the eastern side of town (very easy to find, with her green hair).
Naylie will want you to deliver a letter to her husband, Fred, who lives in Valentown. You can use the Memory Teleportal at the entrance of Hallowtown to travel to Valentown, assuming you unlocked the Memory Teleportal in Valentown too (and you have a Recollection Orb).
Fred and his daughter live in a large house directly south of the Memory Teleportal (the first house that can be seen when entering Valentown from the world map). Speak to Fred, and give him the letter. He will thank you, and tell you a secret phrase so that Naylie will know he got the letter.
Use the teleport to return to Valentown and speak to Naylie to complete the quest, and unlock the achievement.