@Kaarme
Its just one creature and they are the "generals" of the undead army.
IIRC
They were described as amalgamations of human bodies forged into a spider-like creature in the web novel.
they are basically undead spiders abominations made out of human bodies.
That's why they are called "Arachne" type undead.
You can see another one die on page 2. ( and there was one with armour and weapons in chapter 22.2 on the pirate boat )
There has never been a really good picture of how they look so far IMO, as you always only see the upper body and not the spider-like legs( that they had in the novel).
But ( in the novel ) they are something like this but naked, less huge and more spider-like...
( THIS IS NOT A PICTURE FROM THE NOVEL, JUST SOMETHING SIMILAR LOOKING-ish)