"the flight from the Alone to the Alone, of the Dark Night of the Soul and of the final ecstasy in which the creature becomes one with the Beloved"

The flight of the alone to the Alone is part of the fourth state of consciousness, the mystical marriage, the union of God and His creature. The Dark Night of the Soul is a metaphor for a person's spiritual desolation.

W.H. Auden would not write "Brothers who when the sirens roar..." until 1932; he is clearly familiar with these concepts:

Dare-devil mystic who bear the scars

Of many spiritual wars

And smoothly tell

The starving that their one salvation

Is personal regeneration

By fasting, prayer and contemplation;

Is it? Well,

 

Others have tried it, all delight

Sustained in that ecstatic flight

Could not console

When through exhausting hours they’d flown

From the alone to the Alone,

Nothing remained but the dry-as-bone

Night of the soul.