Two of the party members had trained escape artist.
That would, under normal math, give them a slightly lower than 50% chance of escaping the tentacles' grapple each round and then taking a 5' step. And they were how far from the edge of the solid fog?
Plus, in order to even have that 45% chance or whatever, they would have to be in no or light armour, i.e. they aren't frontliner classes, i.e. they don't have many hp, i.e. they're going to die to teeth + tentacles a lot faster than the platewearers.
The point is not that the paladin didn't have a ranged weapon or whatever. Like I keep saying to Sunic, killing someone because herp derp you want them to optimize more isn't good DMing. If you want them to be X powerful, you can just give them twenty levels instead of 10, and a bonus to their stats and damage and crap. You are the DM. You can do that. Optimizing a character is a skill that has a place in DnD but it's not something that separates 'good' from 'bad'.
Obviously the party did have ranged, but they were the people who couldn't escape the fog/tentacles. Which is just more fuel for the one point I made and that everyone on the side of 'herp derp they deserved to die what idiots' keeps not addressing - 'rocks fall, you die' is NEVER a valid reason to kill the party. It's just bad, lazy DMing. If you can't find a way to challenge the party without killing them, then you need to sit down and figure out why, and then fix that problem. Because it means the game is only playable if the characters either don't get challenged or if every encounter or every 1/3rd encounter or 1/10th encounter, something is going to instawin all over their faces and kill them utterly and it's roll new characters time. And that a good storyline does not make.