A poison

[heslopian]

I've been watching a lot of violent sci-fi films lately, so at first I pictured this narrative as a battle between starship troopers and poisonous aliens But it seems like it's a metaphor for depression, and if that's so then it's subtle and very evocative. I do have suggestions, though:

Plunging deeper for a reason, reaching for the Void,
For the Void has claimed a deeper power through a poison,
Poison that was bought upon a fancy, and I call upon the Name-- Whose name? The Void's? As "Void" is capitalised, I figured that that was it's name.
Save her from the merciless Void, This verse is overhwelming, partly because of the odd syntax. "Claimed a deeper power through a poison" feels inelegantly worded, for instance. Would something like "for the Void has used poison to accelerate its power" be clearer? The adjective "deeper" suggests a previous power that you haven't described, which may be why the line feels inelegant to me.
"Bought upon a fancy" feels like a tortured way of saying "bought on a whim"; though the latter may be a cliche, I'd go with it instead, as it's much more efficient.

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, Again, whose name? God's? It clearly isn't the girl's, because "name" isn't capitalised when used in connection with her.

Claimed upon my heart, she had--her poison claiming her-- Who or what had claimed upon the narrator's heart?
Claiming treasured friends, this poison had,
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, I like the use of "asunder" in this line.
Dashed against the stones, of which these broken bones, "Of which" suggests a new thought which is never resolved. "Of which" what?
Laid upon a heap of weeping souls, Nice, macabre image.
Weeping for the mercy cold. What does "mercy cold" mean?

The poem could do with more images and narrative clarity: what is happening, when, where, and why? The central metaphor is good, it just needs to be made clearer, and strenghtened with poetic grounding (imagery, metaphors, similes etc.). All critique is JMHO. Thank you for the read