This Dark Form is what your sim character turns into when he/she does any vamp activity like biting and stuff. When hovering your mouse over the CAS sims head icon, there is a two-toned (half-black/half white) character symbol above that can give your Vampire an alternative so-called Dark Form. Now I'm interested, but I'm getting frustrated by how difficult they made it. I never figured it out, and wasn't interested enough in the game to learn.

Then you can move him/her anywhere, just simply delete the other Sim in CAS before you leave CAS. By Default the game will use the outfit your Sim wears everyday as the dark form outfit. Also setting it's outfits when in dark form. Don't forget to do those things for it, too. Then at bottom in left you will see where to click to make the dark form. Make sure to set his/her facial details.including teeth and ears. Make your normal vampire disguise as you want him/her to appear daily. Then click the Add Sim button scroll down where you see how to make an alien and the vampire option is there. You will delete him/her before you leave CAS.

I was not seeing how to create a dark form yesterday too, but it's easy peasy now.

Sims 4 how to make vampire