It's total RNG, but I'm gonna lean towards the "Bad" side. In 520 paragon lvls Season 1, I never found one. 370 in Season 2, I've found 2, both from Kadala. I have 2 other friends that have around the same play time as me in Season 2 (about 90 hours) and they both have never had one drop. One of them got theirs from Kadala.
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It's total RNG, but I'm gonna lean towards the "Bad" side. In 520 paragon lvls Season 1, I never found one. 370 in Season 2, I've found 2, both from Kadala. I have 2 other friends that have around the same play time as me in Season 2 (about 90 hours) and they both have never had one drop. One of them got theirs from Kadala.