This is supposedly a 4-7% drop rate? at lvl 10 in a particularly dangerous cave.
me and 2 others, with open loot killed well over 100. and still didn’t get it to drop. Either this quest is very bugged. or that drop rate is a complete lie.
you’re not looting right, and one of you had it once already.
I never had this one drop, and I always kill all those mobs several times, as they killed my very first Horde HC 
It takes time for these to drop. I had to kill about 5o of them. Ive also gotten lucky a few times and had it drop on kill #1
The last time I made a HC horde I think I had to literally clear the entire cave before it dropped. Other times it drops off the 3rd mob.
The luck swings both ways.
I got it yesterday after killing about 8 mobs.
You don’t understand probability. 4-7% drop rate isn’t for your personal drop rate. It’s for everyone, on all servers, on all regions. Last time I did it I got it on the first kill. Now to make that probability go from 100% to 5%, someone needs to loot 500, or 2 people need to loot 250, or 3 people need to loot 166-167, or 4 people need to loot 125, or 5 people need to loot 100, or 6 people need to loot 83-84, ect.