I mentioned these in the "Ken vs. the Pizzaplex" arc, but I just figured that I would add more context.

Sendoff Marks were used in the very early days of Ninjago by preachers and doctors. They could either be seen as a blessing or a curse, depending on which Mark was used. They were meant to be carved into the brows of those who were fatally wounded or sick, or already deceased. Doing so would ensure that person's safe deliverance to one Realm or the other.

For the Mark below, it's a simple circle. It appears open, promising, perfect - or rather, the promise OF perfection. Those who were blessed with this Mark would automatically pass over into the blessed Realm of the Departed. It was typically made with the dominant hand and was bestowed upon the hard working members of society.

However, if someone were to slash a line through the circle, the Mark becomes a curse, and the person automatically passes over to... the OTHER Realm.

The Cursed Realm.

The slash represents perfection ruined, or the promise of perfection taken away. It was typically made with the non-dominant hand, further symbolizing chaos, and was bestowed upon those with unsavory qualities. Criminals of the most disgraceful crimes were most commonly dealt this sentence.

However, official use of the Sendoff Marks hasn't been utilized in Ninjago's culture in several millennia, and for good reason. It was eventually discovered that the Marks had the power to send one who wasn't even mortally sick into the Realms, and even more criminals started using the Marks on their victims. In response, Ninjago residents started to leave the fates of the deceased in the hands of the Realms. Most mentions of the Sendoff Marks were collected and destroyed to ensure such heinous acts would never be committed again. The last remaining documents detailing the uses of the Sendoff Marks lie in the extensive library of the Monastery of Spinjitzu.

First Spinjitzu Master forbid that any of the Masters find them...