The line is from Valery's La Jeune Parque. 'Le' cannot refer to 'ombre' since it is feminine. And I doubt it refers to some general thing such as 'reality' like in the sentence 'it rains'. So I'm basically clueless here. Here's the full context:
L'esprit n’est pas si pur que jamais idolâtre
Sa fougue solitaire aux élans de flambeau
Ne fasse fuir les murs de son morne tombeau.
Tout peut naître ici-bas d’une attente infinie.
L'ombre même le cède à certaine agonie,
L'âme avare s’entr’ouvre, et du monstre s’émeut
Qui se tord sur le pas d’une porte de feu.
And while we're at it, let's go ahead and tackle 'entr'ouvre
because I don't get that either. Is 'ouvre' an adjective, preposition, what? I would think 'entre' would be the verb but hey maybe I'm wrong. It was translated as The clutching soul half-opens and is moved
but I don't think that is a literal translation.