Here is what OG D1 players had to do for the Thorn before it was a drop or purchase from Xur
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DESTINY WIKI
A Light in the Dark (Year 1)
For the quest in Year 3, see A Light in the Dark (Year 3).
The Weapons of Sorrow were born from forbidden use of the Hive's dark power. We thought them eradicated.
But recent developments suggest one of these weapons may still exist in the Hive's Summoning Pits.IN-GAME DESCRIPTION
A Light in the Dark is an exotic weapon bountyultimately rewarded the exotic hand cannon Thorn. The bounty could be obtained from the Vanguard Mentor Missive, but was made unavailable in Destiny: The Taken King.
Tasks
Complete The Summoning Pits on the Moon.
Earn 500 points by killing Hive enemies on the Moon.
A normal enemy is worth 1 point, a Major is worth 20, and an Ultra is worth 50.
Use Void Damage to defeat Guardians in the Crucible. Death will slow your progress.
Requires 500 points. 5 points are gained per kill, 2 lost per death (meaning that progress is made as long as the average ratio of void kills to deaths is sustained above 0.4).
Kills must be made with a weapon or skill that does Void damage. Assists do not count.
Obtain an Infusion of Light from The Speaker.
This will cost one Mote of Light.
Talk to Ikora Rey.
Kill Xyor, the Unwed.
Xyor is found in a level 26 instance of The Summoning Pits that is only available while this phase of the bounty is active. The Epic modifier is enabled, and matchmaking is not used.
Phogoth must be damaged enough to trigger waves of enemies but not killed until Xyor appears. She is included in the enemies dropped by the third Tombship.
Return to The Speaker to claim Thorn.
English
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I loved that quest, did it before Christmas 2014. I used a void fusion and lmg. The good old days :)