1. Where is the key to get into the mind flayer area in the sewers in the temple district?

2. Does anyone know what "Festule the Alchemist's Potion" is for? I found it on a book shelf in the temple of Helm.

1.) I don't remember needing a key
2.) there is no use

There is a hidden door, that can't be opened with lock pick or knock.

Spoiler
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You need the Sewer Key, which is loot from Tazok from the Windspear Hills quest.

If you have Mazzy on your party, go to the southern entrance to the Copper Coronet for a quest in which that potion is rather handy

Wheres Tazok and how do you get the key from him?

Dragon Reborn, read Dundee's post again and use your common sense.

Spoiler For Tazok's location...
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Tazok is in the dungeon in the Windspear hills area. After passing through the orc archer gauntlet, you'll hit a hallway. Take the door going south off of it. You'll hit a circular room, take the passageway going north off of it. You'll see a room to your right with a door, open door you'll see a winding set of stairs ending in another door. Tazok is behind that door.

Vily

I must have gotten it and dropped it somewhere, thanks anyway.

Is there any way to get it back without searching the entire game.