You receive the tattoos by using Will spells. The more you use Will the darker they will become.
“The world breaks every one and afterward many are strong at the broken places. But those that will not break it kills. It kills the very good and the very gentle and the very brave impartially.”
If you search for "tattoo" you'll find other threads on this subject.
SWC
Sir Edmund:"Should you obey the lord who asks you to put a village of innocents to the torch? Is that chivalrous? Is it noble?" Me:"It's a great way to get promoted, I know that much."