The Omega
According to the text books, breaking the bond between an Alpha and his Omega results in the death of the Omega. When John's Alpha, Professor Moriarty, is savagely murdered nobody expects John to survive. Still, here he is, alive and kicking - trying to start a new life, while clearing his name, and keeping unbonded Alphas at bay.
Omegaverse with mentioning of rape/non-con and past abuse. None of this between Sherlock and John.