It all happened in the last 30 minutes of the session. Just a series of unfortunate rolls and a couple bad decisions.
We had been exploring an abandoned underground forge (very much like the Dwermer caves in Skyrim). While exploring the lowest level we came across a long circular corridor without any doors and one of the party detected large amounts of magic from behind the outer wall. We found a series of hidden levers that were all booby trapped (blades, poison gas, fireballs, magic disperse) and after at least a half dozen we were fairly battered (series of bad rolls leading to damage). Two of the members used Dimension Door to get through only to find an army waiting on the other side. One was killed by the army and the other escaped back to us. She decided to go back for him and was successful. After reviving him, he said he saw treasure chests and decided to cast Shatter against the wall to go get it (bad decision). The explosion killed 3 of us (another series of bad rolls), and the other two managed to get far enough away to survive only to be trapped on a failing metal stairway over an abyss that we had crossed the day prior (more bad rolls). One fell to her death and the other (a player who just joined us for this session) managed to transform into a lizard and scaled the wall over to the door where she was subsequently killed and eaten by the group's guard beast (a large Emu-like bird with teeth).
The only part that I was bummed about was the fact that I really liked the character build I had going. I managed to come up with a new character with similar attributes that ties into the story really well.
First character was a Half-elf rogue. I used Loki as the foundation. Basically a smooth-talking con-man with illusionary magic. New character is a younger woman, but also a half-elf rogue. She is an orphan and grew up on the streets as a pick-pocket until she tried to steal from my first character several years ago. He caught her and recognizing her talent he decided to teach her. When he went missing several weeks ago (by joining the campaign), she set off to go find him always being a couple days behind and just missing him. She has tracked him to the entrance of this forge when she hears a loud explosion...