They are actually very large and they burrow so your scenario is good, they live in the mist because their bodies are very sensitive to light and heat, they will be able to swallow you whole but not before releasing a sticky mucus that will lock the body up even bones, you wont die if your swallowed as your already undead but you will be trapped, They are very large, white or light grey with a soft, slimy exterior your sword could hurt them but it would have to be a stab as slices while not connect due to the nature of their skin (slippery). However it will take many, many stabs to kill one and they typically move in pairs or if your unlucky threes. Need any other information let me know.
P.S. Based on how you have developed your story, you are most likely about to scale one of the lowest points along the southern peak that borders the Mist Strewn Forest, after about a third of the way up it will be a sheer cliff face; that will be just under the treeline for the smaller tree, it will be very misty your view will only be over the southern most portion of the Forest. (Hint: you might however see a visible landmark in the central area buried in the mist, probably north east of your position *whistles nonchalantly*)