(…) There threatening stared
those dreadul shapes, in hatred both,
false Draugluin and Carcharoth
when, lo! a marvel to behold:
some power, descended from of old,
from race divine beyond the West
sudden Tinúviel possessed
like inner fire.
I often think about these verses from “The Lay of Leithian” (3805-3812). One can wonder if by 'race divine' is meant Melian, Lúthien's mother, herself of the divine race of the Ainur, or the Valar, still, at that time, watching over and acting in Middle-earth. I'm inclined to the opinion the author meant the Valar: it happened just before Lúthien and Beren entered the dungeons of Morgoth the Vala. Of course, we know from the same poem that Morgoth wasn't able to penetrate Doriath, protected by the Girdle of Melian, and in “The Silmarillion” we read that Melian was powerful enough to stop Ungoliant. But Lúthien earlier used her magical powers while escaping from Doriath and transforming Beren and herself into foul beasts. Here, in Angband, she was to do something greater than anybody else would dare, in some way she overpowered Morgoth and regained one of the Silmarils. That power possessed her suddenly, so she didn't have it before.
Still, the topic is open for discussion, one of few unclear passages in Tolkien's works.