I'm re-reading the manga right now. During the Eclipse, we see the brand placed on a number of hawks, each in a different place (link to the panel):
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Casca gets it on the left side of her chest, roughly over her heart
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Guts, of course, has it on his neck
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Judeau gets his on the palm of his left hand
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Pippin has it on his forearm
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Corkus gets his on his forehead (just like the bodies below the tower of rebirth)
I can make an argument for each of them to have gotten it on whatever they most rely on, except for Guts, I can't make a concrete argument why it's his neck.
But what I'm really interested in is this: can we learn anything about the bodies from Gaiserics kingdom from this? It might be that the population of his city wasn't made up of fighters and physical people at the time it was sacrificed, but of thinkers, or at least people who rely more on wit and cunning than on strength (positive or negative). This would make sense considering the technological and social advancements they seem to have made.
What do you guys think?
Another theory, probably more likely, is that the brand is placed on whatever the sacrificer values most of the sacrificee. The sacrifice itself is driven by them, so this would seem more in line with the themes. This would mean Void most valued the mind of the people in the capitol - probably for someone to hear his proclamations of the king's sins?
This leads to even more interesting questions, like did Void really have devotion to something higher as a sage? Or was it most important to be heard by the people, maybe to try and gain good karma?