I swear Xûr isn't going to sell them any time soon.
I haven't seen a boot engram drop from ToC in eons.
If Xûr does sell a boot engram, it's a legacy engram.
Nightfalls pretty much only reward crap legendaries now.
Pretty sure these are the most difficult exotic to acquire.
[i]~Nyahh! =^,.,^=[/i]
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They only dropped once for me. Once.