Explanation of how and why oracles and gorgons can kill a whole fireteam: the vex view reality as a program, that being said the oracles are able to rewrite this program and erase your team from reality. Luckily the oracles can only be summoned by a Templar vex, it wasn't until recently that fallen from the House of Wolves learned to manipulate them. Gorgons are similiar in the abilities they have but gorgons must have you in their sight to do this. For now these vex have only been seen inside the Vault of Glass.