The blade flashed down. Its point drove easily through the skin and flesh until it was buried right to the hilt. The point emerged, dripping blood, from the underside of Fenris' own tattooed left arm.
[...]
Fenris the Feared ripped the knife from his arm. A few last drops of blood spattered onto the tiles, but that was all. There was no wound on the tattooed flesh. No mark at all.
- The Blade Itself, Part I, An Offer and a Gift