Youth and death

What hast thou done to this dear friend of mine,
Thou cold, white, silent Stranger? From my hand
Her clasped hand slips to meet the grasp of thine;
Her eyes that flamed with love, at thy command
Stare stone-blank on blank air; her frozen heart
Forgets my presence. Teach me who thou art,
Vague shadow sliding 'twixt my friend and me.
I never saw thee till this sudden hour.
What secret door gave entrance unto thee?
What power is thine, o'ermastering Love's own power?