The Frowning Skull artifact can be found in Núrnen Fishery, in the north-western part of Sea of Núrnen.
Unhappy Skull, First Age
For a thousand years this skull has been a sacred relic of the most feared Orc Assassin cult within Mordor, the Death's Head. They hold it as a symbol of their glorious future free of the scourge of Man.
"Please… my wife.
Yes, the Uruk have told me you do go on about her.
Tell us what we need to know. Where are the bandits? Where are your friends?
I need to see her! I need…
Yes, your wife. Again. It has been a long time. And while the physical pain of your torture will pass, you will always hurt inside. The loss is like a gaping wound…
You have suffered quite enough torment, and it is time for your misery to end. Your wife is waiting.
Damn you. They're in the woods. The hidden woods.
Take him away…
But you said…
Yes. Our promise. You will be reunited with your wife, now long dead. Know you have betrayed your friends and doomed yourself over a distant memory, a phantom. And when you are reunited with your wife, know that we will also carry your skull as a trifle and a reminder that our will is irresistible. Now, go.
Your wife awaits."
- Based on the Memory Audio and the fact that the skull is from the First Age, the skull is most likely that of Gorlim, one of Barahir's bandits that has been a thorn on Morgoth's forces in Taur-na-Foen after the end of the Dagor Bragollach. Gorlim betrayed the bandits to Sauron in exchange of being reunited with his wife. With those exact words, Sauron revealed that his wife was already dead and that they soon will be reunited, putting Gorlim to death cruelly.