Hi Cathy.
Well done with this SQ mused from a Times Ten!
I like the idea in this - that there is always something evil lurking, waiting for angels to take their attention elsewhere. Of course, the 'dark angel' theme here fits well with 'nightmares'.

Although Swap Quatrain params state not to repeat the same rhyme scheme, poetic license is all good here (and it's not exact so what the heck).
Please take or toss these few items below as you wish.
Cheers
~Cleo
Sweet angels weep in ancient dreams
as thoughts
presage [portend] prophetic theme.
Dark velvet eyes who fear to sleep;
in ancient dreams, sweet angels weep.
(In L3, I think ‘who’ is throwing me? Could you say:
Dark velvet eyes dread Evil's (or Satan's) sleep; )As midnight drapes,
the [dank] chambers reek
of shadows weary as they seek
lost souls who drift in moonlit crepes;
(Interesting use of crepes)
the [dank] chambers reek as midnight drapes.
When comes the morrow
[-] hearts implore;
yet winds still whisper
Nevermore!
In silent grief and gripping sorrow,
hearts implore,
[w]hen comes the morrow?