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The equipment piece is Bloodbough Cowl, Ilvl 420. Ask Mr. Robot is recommending the use of the azerite power Death Throes for a Discipline Spec in order to optimize HPS. WoW will not allow enabling of this azerite power. I have chosen Blightborne Infusion as that azerite power and Sudden Revelation azerite power are the only two available for Disc.
I’m not that familiar with Disc, but looking at Icy Veins, Death Throes is indead a good azerite power for that spec, and does work for it. You need to respec to Shadow first, choose the power, and then you can switch back to Discipline.
A similar thing happens with Holy Paladins, where a ret azerite power, Light’s Decree (which extends the duration of Avenging Wrath) is really good for one of their builds.