The Tree of Pain is a symbol of Fairy Magic. The presence of this mark in the one's arm is indicated the person is tagged and will become a victim of fairy magic.
It's unknown if the mark also the symbol of The Hollow Forest of Arkhmoor in Avalon, where Gilda originated from.
According to the lore, it was used by the Ancient Celts for magic rituals and casting spells. The Tree of Pain is used as a hex to maim or kill the subject bearing the mark for the uninitiated. It symbolizes pain, both physical and spiritual. For the initiated practitioners of Celtic Fairy Magic, the Tree of Pain is a powerful protection symbol that served to bind the tribe. The Tree of Pain symbol had been passed down to the ancient Celts from many centuries.