machineelf
Explorer
What level are they? Any wisdom you can impart about running a Kraken from watching it?
I believe the danger level of encounters is very DM dependent. For a long time when I was first DMing, I was very concerned about not making things too difficult for my players. I was afraid of creating a TPK. So in a lot of subconscious (and sometimes not-so-subconscious) ways I would go easy on my players. My monsters were not out for blood. Cut to a couple of years later, and I am much more sadistic.

So my philosophy now is, I don't want my players to die. I want them to have a tough challenge but ultimately prevail. But the monsters and NPCs they fight definitely want them to die. They will fight tooth and nail to survive themselves. When I prepare for combat scenarios, I make sure to be fully aware of all the monster's abilities, and I make sure they use them as effectively as possible.
I say all that to say, that had the old me DM'd a fight with a kraken, my 13th level players would stand a real chance of beating it. If the current me DM's a fight scene with a kraken, 13th level characters would get eaten up (literally). I would say it would be a challenge even for 16th and 17th level characters, the way I run it. Keep in mind that the kraken is very, very intelligent, and they are most likely fighting underwater, in the Kraken's territory. The kraken is going to be able to blind the characters with ink clouds and swallow them whole. Things can go down hill for your characters very quickly if that happens.