Like a lot of things in Black Panther, Killmonger’s scars too are derived from existing customs in Africa.

In the movie, the primary antagonist explains that he has one scar for each of his kills, but he doesn’t explain how.

There is a tribe in Papua New Guinea, the Chambri with a population of ~1000 people that is privy to a weird ritual. The Chambri believe that humans evolved from crocodiles, and the animal has a special place in their mythos. From this derives one of the most painful initiation rituals for young men - intentional scarification to resemble the skin of the water-beast.

A good read on the topic is Rituals of Manhood.

Frequently, when two of the initiates belong to different initiation moieties, they are scarified simultaneously, each by specialists belonging to the opposite moiety. Whenever possible, an equal number of initiates from each moiety should be scarified, for only then will each moiety's ancestral crocodile, who is understood to be responsible for “eating the backs of the initiates,” return to a peaceful sleep after the ceremonies are completed.
The process is quite painful.

To pay tribute to the creatures from which they evolved, the boys of the tribe are taken to a spirit house and kept there for six weeks before the initiation process happens.

Then tribal leaders make deep, 2cm-long incisions across the bodies of young males - ranging from ages 11 to 30 in some cases, according to National Geographic.

This excruciating process - which occasionally ends in death - is meant to demonstrate discipline, focus and dedication, and the only way the boys are allowed to numb the pain is by munching on a plant with healing qualities.

After this, the boys then stretch out in front of a fire so smoke can blow into the deep nicks, while clay and tree oil is inserted into the lesions to help them maintain their raised scale shape.
The scars are raised and kept that way by inserting a mix of “clay and tree oil” in them. It wouldn’t be far fetched to say that Killmonger did something similar.

And they are not the only ones to do this.
Don’t worry, Killmonger’s personal significance the ritual (as far as I know) is exclusively his own invention. Just to show how addicted to killing and murder he is.

This ugly thing would have been considered as the dopest thing ever.

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