[Nerwen]

Explanation:

Twins are we
Imbued with his hate -both made by Eöl out of meteoric iron and said to contain his "dark heart".
Our lives are very different
Yet we shared the same fate -perhaps in the sense of being ultimately abandoned? Or having all their wielders die?

One remained in the fires
Where the turncoat was slain -Anguirel, left in the ruins of Gondolin.
The other inside the earth
Till the time comes for it to rise again -Anglachel aka Gurthang, buried with Túrin.