A poison

[R.C. KITCHENS]

Plunging deeper for a reason, reaching for the Void,

I would suggest a line about the "reason"
just an example- "plunging deeper into the void because of scotch"
"plunging deep into the void because etc.."

For the Void has claimed a deeper power through a poison,

I don't know if "for" is a good starting point and you use "deeper" twice in the first two lines. Maybe a substitute?

Poison that was bought upon a fancy, and I call upon the Name--
Save her from the merciless Void,

I dont understand "poison bought upon a fancy" the void is probably
a giver of free poison, that is what I take, but its bough upon a fancy?
I'd suggest using a name "I call upon a name" "I call upon Sarah" or another name.. it just add's a little detail.

Reaching to my friend as the poison called her name,
Poison that was killing, leaving nothing but a shell,
Called upon the Name, as I called upon her name,
Nothing from the Name, no mercy from the Void,

This stanza above is hard to understand, the "I calling on the name the void calling on the name, nothing from the name. Believe a good metaphor could be used to show how "no mercy from the void" really is.

Claimed upon my heart, she had--her poison claiming her--
This is where I see " the bought fancy" "her poison claiming her"
I don't think "her poison claiming her" reads very well. Maybe a rework on the line.

Claiming treasured friends, this poison had,

the main focus is on a girl, now it is on multiple friends. next line proceeding it is back to the one friend. I would stick to "friend"

Slowly killing, taking my treasured friend,
Now I weep for the mercy call,

All my dreams are now asunder, all my hopes have been dashed,
Dashed against the stones, of which these broken bones,

Dashed is the wrong word to use. and "dashed against the stones, of which these broken bones," makes no sense. "The stones thrashed against broken bones" would sound more correct.
Laid upon a heap of weeping souls,
Weeping for the mercy cold.

The transition from a friend being poisoned to your own turmoil in the last stanza doesn't go well. I think you could spend some time with this and try to work in some better word choices and metaphors, imagery. I think a little focus on the story could provide for better story telling. Just my opinion, It does need some work.