It's a proper mermaid skirt!
It has been a loooong time since I tried this look but this particular fabric, a beefy, polyester/wool/viscose/lycra ponte knit, was just asking for it!
It drafted as a six panel skirt but I added a center back seam for fabric economy. I started out with the same shaping on all the panels (just more flare and a bit more length towards the back) but ended up taking it in at the side waist and at all the back leg seams between hip and knee.
This skirt actually tapers significantly towards the knee and only flares out from there. I would never do this in a woven fabric but this is a knit, it has stretch so something approaching a true "Morticia"-look is possible.
Drafting a pattern like this is fairly simple. I didn't use any instructions (I know how to do stuff like this by now) but you could try out different ways of drafting a skirt like this using this tutorial from Studio Faro.
Because of the fabric, I didn't even have to put in a zipper. I drafted the skirt to extend to about 4 cm above the waist and sewed 4 cm wide elastic to the top seam allowance, creating a nice, snug internal waistband (which I can still get over my hip because it stretches...)
I haven't really worn the skirt yet but I when I tried it on with this tweed jacket which I made many years ago, I just loved the look!
(in fact, it really reminds me of an Azzedine Alaia design of which there is a picture in that nice big book from the Kyoto Fashion Institute... I just don't own the book and can't find that picture online right now. Of course, the designer outfit is much more extreme, but my stuff kind of looks like a wearable light-version of the same idea...)