From Destinypedia, the Destiny wiki
Thorn is an Exotic hand cannon and one of the Weapons of Sorrow. It is most likely named after the thorn-like spikes on it. The weapon is sleek, dark black, with many spikes or "thorns" adorning the sides, facing the front of the weapon. The projectiles shot from it also resemble giant daggers or "thorns". It was acquired via the Exotic bounty, A Light in the Dark. Thorn returned to Destiny 2 during Joker's Wild and is acquired through a hidden Quest. As of Beyond Light it is only acquirable through the monument to lost light.
Also see: Dredgen Yor
Thorn was once called Rose, and it belonged to the Guardian Rezyl Azzir. Once a noble hero, Azzir eventually fell to Darkness, becoming Dredgen Yor after looking to the Hive to try finding more ways to defeat the Darkness. He was soon consumed by the Darkness, and he and his Hand Cannon Rose began wantonly murdering innocents and consuming their Light. Ever since, it has been hungry for the Light of other Guardians. Whether before or after it was built, Thorn became bound to Xyor, the Unwed; killing her removes the link and enables Guardians to safely use Thorn without risk of corruption.
After Dredgen Yor's fall, cleansed replicas of the Thorn were created by the Shadows of Yor, a group of Guardians who idolize the fallen Guardian. Their activity raised enough concern for Shin Malphur, Yor's killer, to take up arms against the Shadows. A year after the Red War, a Shadow of Yor known as Callum Sol was found and killed by Shin. The Young Wolf would find the spot of the Shadow's final death, retrieving weapon parts from his remains for The Drifter to utilize to create a new replica of Thorn known as Malfeasance.
It is later revealed that Shin's killing of Callum Sol was in fact a trap for people who are vulnerable to falling to the Darkness, and that Shin Malphur is actually a member of the Shadows of Yor himself.
During Penumbra, The Guardian found Yor's original Thorn. In their hands, the Thorn would first be cleansed to reveal the original Rose underneath, and then extensively modified and charged with Light to become Lumina.
At an unknown date, a Warlock named Jana-14 obsessively researching the Thorn and other parts of Yor's knowledge managed to reverse-engineer the Mark of the Devourer and create an explosive, contagious version of the poison, applying it to a pair of gauntlets named the Necrotic Grip. Sometime during the war against the House of Salvation, the Guardian would obtain the Grips, and would discover its ability to synergize with the Thorn itself, enabling the gun's rounds to take on the traits of the Grips' poison.
Thorn, like every exotic in Destiny, has a "signature perk". Thorn's is called "Mark of the Devourer", which allows rounds fired from the weapon to pierce the target and cause lingering damage over time,  which, in PvP, could potentially prevent a fleeing Guardian's shield from regenerating quickly enough for a kill-shot to be delivered. This perk has since been nerfed from its Year 1 iteration.
"The Weapons of Sorrow are not the endgame, but a road map. Each evolution, every advance in the delivery of pain and the mastery of destruction feeds the Hive's hateful weapons research. They will map every scream, harness every aggression, until they understand every method by which to ravage the hearts, minds, and flesh of man. And in doing so, they will turn us against ourselves—feeding our lust, our greed, our fear, until we become a threat unto ourselves like none we could imagine. So, wield these, angry reaper. Strive to know the darkness in your own heart. Walk in the shadows of fallen heroes. And know that you are an enemy of hope."
While not listed, equipping the Necrotic Grip Warlock Exotic arms will cause the armor's poison explosions (normally triggered through melee attacks) to trigger when Thorn kills a target through direct damage, greatly increasing its crowd control capabilities. As with the original Exotic perk, enemies that die to the poison damage from an explosion will themselves explode as well, spreading the contagion further.
List of appearances