I used a knife to slightly cut open a finger.
If someone else tried to stab me, the shield would respond. But it was unresponsive if I ever harmed myself.
Furthermore, it won't activate outside of battle.
I waited for blood to ooze out, then dripped several drops into the saucer.
The slave trader used a brush to soak up the ink, then he tore the slave's clothes down to her waist and painted the stigma on her body.