Industrialization

I bet it is human magic, and you, human, will help me trace it!
I have a potion that allows seeing magic. If there are evil charms in the water, you will see them. Just find the source of the sorcery and bring me some samples of it. I will try and find a way to deal with it.

Drink the potion, and you will start seeing the traces of evil magic in the lake.

Here it is! Do you smell it? Ah, sorry, you can't smell under the water.
It makes me dizzy, and also makes me want to tear someone apart... Ah, what a nasty thing!