written in blood

adjective2

1. •
   (figurative) Having arisen out of tragedy.

   • Safety rules are written in blood...they exist because someone was hurt. — 1997
2. •
   (figurative) Incredibly harsh or strict; brutal.

   • The Colony was ruled during that period by laws written in blood; and its history shows how the narrow selfishness of despotic power could counteract the best efforts of benevolence. — 1874