Looking at bungies new armory website for destiny the description of where the gally drops includes xurs inventory. If I'm not mistaken it was said before that it was removed from his inventory https://www.bungie.net/en/Armory/Detail?type=item&item=1274330687&itemname=Gjallarhorn
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Technically he has sold it before. It also says he can sell Montecarlo, Hawkmoon, Thorn, Bad Juju, Invective, Plan C, etc. I know Hawkmoon and Montecarlo are PS exclusives so Xur can't sell them until the PS exclusive deal ends. I am more curious about Thorn, etc. I thought the only way to obtain these weapons was through exotic bounties.