Good evening, Sir Priest, and so late as you ride,
With your mule so fair, and your mantle so wide;
But ride you through valley, or ride you o'er hill.
There is one that has warrant to wait on you still.
Back, back,
The volume black!
I have a warrant to carry it back.

What, ho! Sub-Prior, and came you but here
To conjure a book from a dead woman's bier?
Sain you, and save you, be wary and wise,
Ride back with the book, or you'll pay for your prize.
Back, back.
There's death in the track!
In the name of my master I bid thee bear back.

'In the name of MY Master,' said the astonished monk, 'that name before which all things created tremble, I conjure thee to say what thou art that hauntest me thus?'
The same voice replied,-

That which is neither ill nor well,
That which belongs not to Heaven nor to hell,
A wreath of the mist, a bubble of the stream,
'Twixt a waking thought and a sleeping dream;
A form that men spy
With the half-shut eye.
In the beams of the setting sun, am I.

Vainly, Sir Prior, wouldst thou bar me my right!
Like the star when it shoots, I can dart through the night;
I can dance on the torrent and ride on the air,
And travel the world with the bonny night-mare.
Again, again,
At the crook of the glen,
Where bickers the burnie, I'll meet thee again.

Men of good are bold as sackless
Men of rude are wild and reckless,
Lie thou still
In the nook of the hill.
For those be before thee that wish thee ill.

Sir Walter Scott

http://www.poemhunter.com/poem/to-the-sub-prior/