Nezznar's Journal and notes

He's from Menzoberranzan, contains a rough map with rough directions to the city in the Underdark.

Also map of Wave Echo mine.
Journal contains origins of Nezznar, of house Baenre. Contains info regarding leads to  other places on Sword Coast.  One is the medusa's tomb, as yet undiscovered.

Concerns dominated by being summoned for judgement by his superiors, priestesses of Lolth, and of failing his test and being turned into a drider by the yochlol demons. Lots of praise to Lolth follows.

Yochlol: Demons. The "handmaidens of Lolth" who live in the Abyss. Drow high priestesses can summon them with rituals. They can appear as columns of gas, or an amorphous column with one eye, or even a giant spider or beautiful female human or elf. They can contact their goddess Lolth telepathically.

Journal entries are mostly Nezznar moaning about the light, having to live in caves instead of real buildings, and his difficulties in keeping notes. Moaning about overland travel. He would kill for a massage.

Other general information about the drow from the Underdark comes through in the journals;

That the drow worship Lolth and believe they are destined to be rulers of darkness.
That their magic equipment loses their magic after one hour in sunlight. Much of the enchanted equipment carried up to the surface became mundane upon hitting the cursed daylight.

He's also moaning that doppelgangers can't  be charmed into doing things, and expresses frustration that they can always read his mind, so he has to be honest. They're too useful, and they know it, and that his offer has to be sincere.

He was sent out on a mission several months ago by superior, Narcelia Baenre (Cleric of Lolth), of house Baenre. He is one of many agents sent out to investigate areas of magical power. Sent with a group of drow to investigate the lost Mines of Phandelver. However, unknown to his peers, two of his servants (Slith and Kheel) were doppelgangers who he promised to pay handsomely for their services, and provide an opportunity  to get themselves and their young out of servitude to the drow in Menzoberranzan.

When they got enough leads, he had his doppelganger's kill the other drow so as to get all the credit.

Used doppelganger agents and magic to strike deals with the Bugbear King Grohl of the Cragmaw tribe (Although Nezznar never went in person, they hate elves). Targor Bloodsword and his sons are thankfully not in charge, as they are more interested in waging war on Phandalin than working with Nezznar and getting paid for bounties and kidnapping. Lord Albrek (Glasstaff) was considered approachable when the doppelganger in Phandalin read his thoughts, and found his true nature suitable. He wanted magical knowledge, and kept asking for Nezznar to teach him the spell Invisibility.  He's only granted him potions so far, but he isn't able to cast yet. He's probably trying to brew them himself with a lab in the ruins he calls home.

When the Rockseeker dwarves found the mine, the thought reading doppelganger's had already read their secret knowledge, and were ready to strike. Giant spiders in the mine responded with affinity for the Drow, a blessing from Lolth.

The last journal entries show Black Spider's attempts to tie up loose ends and the party's disruption. No record suggests he was aware that one doppelganger was captured by PCs. By all accounts, his entries and increasingly frantic handwriting show frustration, his plans foiled by people so... UNCHARISMATIC.

Nezznar also makes notes on the ecology of the wraith in the mine. Worries about it's ability to spawn specters. Several bugbears drained of life, their spirits raised and rained the life out of more bugbears. So, he stopped sending in bugbears. Doubts creatures killed in this way can be raised from the dead. And notes to self "even then, I am more than 10 days travel away from a cleric who would be willing to cast raise dead on my corpse. Perhaps once I could have used Glasstaff, but he's dead now. No, I would rather not pass over to the judgement of the spider queen without more to my name".

Also notes presence of a beholder (he deduces it is one from fleeing bugbear descriptions). Again, Nezznar has not actually fought it himself. It guards forge of spells.

Finally, Nezznar imagines  there is more to the mines, to be uncovered once the operation is secure but that there will be an excavation effort to uncover it all. Hobgoblin slavers should solve the problem.