Skest are strange beings native to the swamps and marshlands of the Lower Land. They take the form of eerily luminescent bubbles, illuminated from within by an unearthly and sickly greenish glow - and if you know your Donaldson, you'll be aware that nothing good ever glows green. They move in utter silence, floating slowly to wherever it is that they want to go.
As one would expect from their bubble-like forms, skest are extremely fragile creatures that may be burst and killed by almost any means - a poke from a stick or a slap from a sheaf of grass is more than enough to put an end to a skest. Despite this, they are considered as highly dangerous for three main reasons:-
i) There seems to be an almost limitless number of them, and
ii) They show no fear for their own preservation, and, most importantly of all,
iii) Their bodies are made of living vitriol.
Yes, the merest brush of a skest's body upon exposed skin will cause a serious burn and getting too close to one when bursting it will reward you with a faceful of highly caustic acid. Best to run, then.
It is unclear whether the skest have any more than the most basic of intelligence and how (or indeed if) they communicate. Having said that, entities far more mighty than themselves have bent them to their will. The Lurker in the Second Chronicles was clearly worshipped in some bizarre way by the skest and as such was able to command them to herd potential fodder towards itself. However, by the Third Chronicles, the Lurker has obviously got bored of the skest - perhaps squiddly-diddly was sick of the mess they kept making on the carpets - and has made them all redundant in favour of surrounding itself with the more vocally kissy-ass Feroce. From a skest point of view, it's perhaps then a good thing that Lord Foul himself has stepped in to employ them as protectors of and carers for barking mad Joan.