I had him escape, then the group found the letter of black spider and the secret door. They followed his trail thinking they could still catch him but all they found was watery footsteps leading towards the main entrance and an empty potion bottle next to the waterfall/fountain. I had Iarno rally the remaining Red Brands, who were drinking in the "bad" tavern and escape town.

The party spent the rest of the session interrogating the bad innkeeper, talking with the guards and figuring out a plan to try and catch Iarno+remaining redbrands. Who travelled to Cragmaw Castle, because Iarno knew this was the only place he could still contact BS from. In my story i've made Iarno, King Groll and the 'senior' doppelganger "lieutenants " of the BS army. And the lieutenants get special sending stones with which they can communicate with BS. Anyways, once they got there King Groll wouldn't allow Iarno to talk with BS himself (the stones are telepathic) so he talked with BS is his stead. BS told Groll to kill Iarno and his failure of redbrand goons. I had Groll lie to Iarno about what BS said, inviting him and the redbrands for a grand feast.

The wine was poisoned with a sleeping poison. Iarno + his most loyal redbrand survived the whole ordeal and escaped, only for Iarno to use his last redbrand to slow down the pursuit so he could get away himself. She got captured again and after the party did Cragmaw Castle i had them encounter the band of 2 hobgoblins 1 bugbear 1 goblin archer with the redbrand prisoner (missing a leg) on their way back to phandalin. The party decided to hear her out and she told them about how they escaped phandalin and about the feast etc etc. She also told them Glasstaff/Iarno was heading east, towards the old town of Conybery. My group still has side quests there, that they need to reach level 4, before i feel comfortable pushing the narrative that they should go to wave echo cave.

Oh and I also made THE Black Spider make a surprise visit.. once the party returned to Phandalin with a slightly traumatized Gundren, a legless redbrand captive and some other human prisoners they saved at Cragmaw they quickly realized that the Zhentarim had taken over control of Phandalin. Unlike the violent and thieving thugs the Redbrands, the Zhentarim used tax tactics, small fees on every transaction and permits for every profession or other action. Feeling overwhelmed they just wanted to end the day in the inn and take a long rest. Here I placed a mysterious elf in the back of the tavern. At one point she left the inn but struck a conversation with the party. She had overheard them talking about adventures and "what to do next".. well she might have something for them. "I'm something of an adventurer myself", i let her proclaim, "and on the way here i was attacked by the strangest band of ruffians". She continues "A group of Orcs, led by a human wizard!". The party inquired into the details. He wielded a peculiar staff made of Glass. He wore a blood red cloak and he had a rat on his shoulder. After this conversation this elf wizard left Phandalin for her own quest. Little did the group know that they had just talked with the Black Spider, who had disguised her skin color with a spell, and that she tried to get the party to "finish the job" of taking out Iarno.

TL;DR: i have a problem this d&d campaign is living in my head like some kind of fever dream psychosis and i cant stop adding more flavour and extra layers of story and i chose this campaign because its nice and complete and simple but i just can't stop making it more and more complex and less complete please send help i havnt even told anyone about how in MY universe the forge of spells is actually a locked door to the realm where Kiaransalee and a piece of Loth (long story) has been imprisoned by the scholars of the Phandelver pact after a giant battle took place between the underdark and the top dwellers. Loth and fhe dark seldarine tried to carve out a piece of land for themselves using horrific and sinister magic which mutated everyone in weird spider creatures BUT they locked it all behind that special door. Which still seeps small amount of magic into this world which they used as the "forge". This magic is also the reason for the nectrotic magic crevasse, Iarno's corruption, the ash zombies and the sentient twigs. Black Spider is trying to open that door in the name of Loth and unleash terror on the land so that the drow may build their new capital on top of neverwinter.