[apelapan]

Most commonly it is pickled herring. I don't think rotten herring is a thing in any broader circles.

Homemade bäsk is usually much better than factory made Bäska Droppar, if you enjoy the taste of wormwood more than just being slapped in the face with artificial bitterness and sugar.

Of course, if you hate the taste of fish, pickled things and spirits in general, you are unlikely to enjoy any of it.

[seasnake]

Surströmming is not pickled but rather fermented

[apelapan]

Also not quite the same as rotten! And, I would hazard a guess that the consumption ratio of inlagd-sill to surströmming is at least 1000:1, maybe 100000:1.