According to the theory, Halbrand will fall in love with Galadriel. She will not reciprocate his feelings, but he will become obsessed. Since she is an immortal elf and he is a mortal human, Halbrand will attempt to achieve immortality. He will do this by acquiring one of the legendary Rings of Power, which will then turn him into one of the canonical Númenórean Nazgûl or Ringwraiths.
All three performers were impressed with the idea. All three agreed that there was definitely chemistry between Galadriel and Halbrand. Baldry called this the Ringwraith theory an “absolutely awesome” idea for a storyline.
So, does this fit into established canon? As Tolkien fans are well aware, the Ringwraiths originated during the Second Age when the “Rings of Power” take place. Sauron created the Rings of Power and gave nine of them to nine mortals, who eventually deteriorated and turned into the Nazgûl.
Little is known about who these people were prior to receiving the rings. In The Silmarillion, it is revealed that three of them were formerly great lords from Númenor. So given Halbrand’s humble origins, it’s entirely possible that he could be one of the remaining six.
The Silmarillion also states that “Those who used the Nine Rings became mighty kings, sorcerers and warriors of old in their time. They gained fame and great wealth, but it turned out to be their death.” It never mentions that one or more of them are looking for the ring out of unrequited love. However, given how many gaps there are in the original canon, it’s entirely possible (via Tolkien Gateway).