This shows Golgatha. Having the skull shown in this way creates the image of what Macbeth + Banquo where trying to do on the battlefield very well. The skull has a cross, which suggests that in their mind,their doings where fine,even right, as they where serving their country + God. The reason this is surprising is because this was the place where Jesus was crucified, meaning that it is the place most against God. They where trying to create this on the battlefield, contradicting themselves.