D is a hunter and is a merchant. His qeustline is useful beacuase if you progess in his questline he will allow you to buy incantation from his and can teleport you to the Bestial Sanctum in Dragonbarrow.
You first meet D at Summonwater Village. You will also find Tibia Mariner, a boss that drops a deathroot that is found around The Lands Between from defeating simalar bosses. You will find D standing on top of a dead body near the closest site of grace (The one next to the pumpkin head guy). After exausting his dialogue and going to the Table of lost Grace he will appear sitting down and you can now buy incantations from him. After talking to him again he will tell you to go the "Beast Clergyman Gurannq. After that a red mark will appear on your map. When you go to the mark there will be a portal.
After going in the Bestial Sanctum you will find a site of grace and a NPC. That NPC is called Gurannq and will give you a quest to get deathroots (i explained it like a second before). When you give Gurannq deathroot he will give you beast related prizes.
Thats it, but there is one more thing. If you give him 3 deathroot he will go hostile, just attack him and thats it