What is the meaning of the tattoo design on Xiao's body in Tomb Raider?

It's a unicorn, a symbol representing his family.

The unicorn tattooed on Zhang Qiling's upper body is greenish-black in color, with antlers and dragon scales, and treads fire and burns wind. It is normally invisible and only becomes visible when the body temperature rises. After intense exercise or close combat, a complete pattern similar to the "unicorn stepping on fire" will appear. (In the internal system of the Zhang family, the head of the family is tattooed with a kylin to represent a "contract" with the family mission.)

In the novel, the Zhang family uses the unicorn totem as their family symbol, and this unicorn tattoo on Zhang Qiling's body is a family tattoo, which only the patriarch gets, and can be said to be the symbol of the Zhang family patriarch.

Expanded profile:

In the Tomb Raider novels, in fact, Zhang Qiling is not the only one with a unicorn tattoo on his body, it is said that the Hmong leader who blinded Chen Pi's eyes with his flying dagger has this unicorn tattoo on his body, and Papa Panma has this unicorn tattoo on his body, too! The original story says that this kind of tattoo is also found on his body. In the original said this kind of tattoo is used by the Miao people to prevent which kind of disease, there is also a saying that this is the map of the Miao village;

Or the map of Zhangjiagulou, tattooed on the body to guide the way for people. There is also a saying that the unicorn tattoo serves to subdue demons and drive away evil spirits, and that the tattoo of the black unicorn tattooed on Zhang Qiling's body can help him subdue demons and evil spirits when he fights with the dumplings in the tomb. About the unicorn tattoo, a variety of claims are particularly cumbersome.