Is it at 0 HP or do they die when they hit negative equal to their Con? I have a 1st level half-elf wizard 1 with 10 HP; her familiar gets 5. If she sends her familiar into combat does it die at 0 or at -11?
negative the familiars con, unless it is an undead or construct familiar. Same as any other creature. There is nothing in the familiar rules that over-rides any other rules about when you die.