“He Took My Left Hand!”

“He took my left hand!”

We know now that Merlin cut off Morgana’s hand and forearm and melted it down to infuse it into the metal parts of the Amulet of Daylight. (Were they already enemies by that point, or was that the final straw that made them official enemies?)

Maybe the hand was supposed to give the Trollhunter resistance to Morgana’s magic?

Angor Rot could not harm whoever wore the Inferna Copula, made of his flesh and containing his soul (though that didn’t somehow stop him from smashing the ring when nobody was wearing it - maybe because that was an accident). Angor’s eye is put in the amulet to counteract Angor Rot’s magic, and the Triumbric Stones (at least one of which was once physically part of Gunmar) are used by the Trollhunter to resist Gunmar’s mind control and create the Eclipse Sword, which is implied by the narrative to be the only thing able to kill Gunmar. 

I suspect the arm in the amulet was supposed to be more of the same.

(Also, there’s clearly part of her arm being melted in that cauldron, but since she screams, “He took my left hand!” rather than “He took my left arm!”, the cut was probably made below the elbow joint. Cosplayers, make a note.)