Ok. Needs specification for which abilities are (Su), which are (Ex) and which are (Sp). I'd guess that Bound to Darkness and Black Reach should be (Ex), with the others as supernatural.

Bound to Darkness- If one already has Tome Tainted Soul one should maybe be allowed another bonus feat from some short list. I'd suggest Great Spell Focus(Necromancy) or any metamagic feat as options in that case. One should also if one becomes undead get the option to perform that swap out then.

Conduit to Torment- why not let them choose which stat to do damage to when they use the ability?

There are a few typos. Loose/lose needs to be fixed. Also, what you call stat is formally called an ability score.

Dark Feeding is potentially abuseable as written if some other PC has an easy way of recovering either hit points or ability damage.

Harbinger of Misery- basing daily uses off of con is interesting. It should probably specify explicitly that they get a min of one use, and that characters without con scores base off of charisma.

"1/day a Mage of the Midnight Coven becoming a beacon to the Negative Energy Plane"- Better wording: "Once a day a Mage of the Midnight Coven may become a beacon to the Negative Energy Plane"

" (Will DC = 10 + Intimidate negates)"- I'm not sure what this means.

By fifth level, victims of a Mage of the Midnight Coven's Inflict spells are racked by a devastating pain, which paralyzes the subject for 1d4 rounds unless they pass fortitude save DC = (10 + MoMC Class Levels + INT mod)
This should be optional. Also, forcing it to be int is going to not be good for casters based on other ability scores. I'd suggest just making the save DC be the same as the save DC of the spell. Also, "Prison of Pain" should be in bold.

I'm a little worried that the class is shut down easily by someone with a death ward. It would be nice to have something like getting to make a caster level check to overcome death wards with their inflict spells, or have them deal half damage through it. Maybe something similar to my Nightlord of Moil's capstone.

Overall, I like it.