This adventure is great. It presents a good challenge for new players, but there is one little problem; if the players are using the pre-generated characters with their backstories, this encounter doesn't work. The halfling rogue is headed away from Phandalin to regroup in order to get revenge on the Redbrands and the dwarven cleric (who is a cousin to the Rockseeker brothers) doesn't need any introduction to Gundren nor his brothers. Also, Gundren and his brothers have already discovered the lost mine of Phandelver, explaining why the two other brothers were in the mine to get captured/killed. If Gundren didn't make it to them with his newly acquired map, then the story for within the mine wouldn't be the same thing at all and the Black Spider wouldn't be already in the mine digging around. Or if the Black Spider used Gundren's map to find the mine, that would mean that the players attempt to rescue him (although heroic and just) would prove pointless in the attempt to find the mine. If the writer of this supplement intended Gundren's brothers to leave Neverwinter first to get to the mine (after all, several days is definitely enough time to copy a map and get a head start), why weren't they at the Dwarven king's tomb helping Gundren? And more importantly, why didn't they wait for Gundren so they could all travel together...? Safety in numbers after all. Anyway, none of this is explained in this supplement, which is why I'm rating it 4 stars instead of 5.
I kinda figured out a work around for this; well, at least for me! I will still try and work in this adventure for when my players set off from Neverwinter towards Phandalin, but instead of Gundren that they rescue, I will have another unrelated Dwarf present who will grant the characters a boon of the spell Aid (+5 temporary hit points each) after they have completely cleared out the tomb and long rested outside of it. This Aid spell will only be necessary if they haven't reached level 2 yet, of course. Then next up for them will be Goblin Arrows!