ToM is a TTK quest weapon not TDB.
The Crux of Crota is to get a weapon called the Necrocasam which came out with The Dark Below.
For ToM please re-read what step you have to do.
Thorn is a year 1 primary weapon which can be obtained as a RNG drop from year 1 activities ( PoE/VoG/Crota) and if he sells it Primary Legacy Engrams from Xur.
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Thorns year 2
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Nope as of this time Thorn is a year 1 exotic only maybe later on in the year a year 2 version may come out. Anyone using it is using 1 with a max light of 170.
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No I’m using it now it’s 400 light