To say the least I was surprised to find that no guide here or via googling could be found for the DLC, all results led to buy pages and obscure Necrophage mentions in other media - So here's my first guide ever, lemme know your thoughts!

This guide shows the 4 seal locations for the Curse of the Necrophage DLC quest. The 'hunt' begins by interacting with one of the four statues after interacting with the coffin in the Necropolis.
(Located in Kyallisar, image courtesy of Lame Digits)

NOTE: Click images to make them bigger.

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Seal of Death Location: Cenotaph: Ruins
Go to location, Kill enemy, get seal.

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Seal of Pain Location: Cenotaph: Spear Pass
Go to location, Kill enemy, get seal.

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Seal of Suffering Location: Cenotaph: Dura'Tyr
Enter the house, go north. Kill werewolf, get seal.

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Seal of Decay Location: Cenotaph: Fallen Eagle Fortress
Once inside the tomb, go to the Fire lasers area and then south, kill enemy, get seal.

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Once you have all 4 seals, go back to Necropolis and interact with the 4 statues - Order does not matter.

This activates the ring puzzle - The solution is seen in shadow realm above the coffin.
Do note that this ring puzzle requires the CORRECT placement of each runic symbol as well.
If you require more aid with the ring puzzle, watch this
Enter portal, bring Necrophage to 0 HP. Kill adds (several waves), bring Necrophage to 0 HP. Repeat twice.

Aaand done. Please rate the guide and comment if it helped you ^_^