@APlusPeace It's probably a reference to a painting "The Ambassadors" by Hans Holbein. The same skull can be seen from a certain perspective there. The chapter's title also seems to suggest that.
I knew about this painting. But still dont get the reason behind the reference. Does that mean something, like close death and etc? Maybe author just saying that skull is cool, isn't it? But thanks for helping)
Memento mori: "Monks and Bible scholars popularized the phrase under the belief that being conscious of your own impending doom would make you behave like a better person." "Hans Holbein painted death as it appeared in life: obscured yet omnipresent. Just as death can ambush us on the moments we least expect it, Only once we become informed of its presence do we begin to adjust our vision and reevaluate what we had previously seen"