Ok so I dont know about the alter of sorrow guns but I'm pretty sure the essence weapons and armor can be obtained as loot drops from the Pit of Heresy dungeon
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YOOOOOO YOU'RE RIGHT!!! You can even get Altars weapons as well from the final boss! Thanks man, I would've never thought about that. Its all listed out on this wiki on the bottom: https://www.destinypedia.com/Pit_of_Heresy (just need Hymn of Desecration to get the Altars weapons). Time to lose my mind in PoH :)
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Glad I could help. Hope you get the drops you need