You've probably seen horrible batches of these nasties popping up in the Free Market entrance...
From what I understand the monster spawn bags are from The Tower Of Oz and they're being used to harass and screw everyone's day up.
Primarily in the Free Market entrance where you can't use skills at all and you're restricted to base level attacks it's almost impossible to kill them because they're so powerful and have so much HP. Lesser characters (below the 150 range) are just wiped out left and right, losing tons of experience in the process.
The only way to reliably destroy them is with giant potions, basically make yourself huge and then just stomp on them, but even then you need a pretty significant range to get rid of them.
Not sure if this issue has been patched with the latest update or not, but if not, be aware that the Free Market entrance may not be a very safe place to be as it can potentially destroy experience gains or make you waste safety charms and the like.
Mmmm, also, I think they have upwards of a billion HP, so even if you can use regular attacks on them they're pretty tough.