The Wheel of Torment staff is bugged. It seems there are atleast 2 bugs, maybe 3.
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Torment applied by the staff does not appear to benefit from Chthonic Fissure nodes Twisted Waves and Grim Tides. Testing on dummies show that torment applied by Soul Feast with this staff equipped is doing SIGNIFICANTLY LESS damage then when applied by Chthonic Fissure.
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Another reason to add to the above could also be that the torment appled by Soul Feast with this staff might also not benefit from the necrotic penetration that the staff provides. Im not sure about this one but considering the difference in damage is 10x I believe both points to be true.
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Chthonic Fissure spawned by the staff also doesnt benefit from the Chthonic Fissure skill tree. Namely Of Gloom and Flames doesnt work. Most likely other nodes dont work aswell.