"WE ARE BUILDING IT FOR KHAAAAAAYYOOOOOOOOOOOOOSSSSSSSSS!!!"
- – Cornholio the Cultist, building unmanly altars for DA BLOOD GAWD!
Skull Altars are dread monuments raised by the servants of Khorne in veneration of their gory master and are formed from piles of violent tributes left in offering to the Blood God's throne. If you know anything about Khorne, you would know that the one thing he hates the most are pansies who spends most of their time building worthless altars when they could go out and let it rip.
Khorne's temple of worship, after all, is in the battlefield, not jacking off in a fucking building. So Khorne knows why GeeDubs decided to violate their own fluff. They are present in both 40k and Age of Skubmar.
They also appear in the second issue of the Marneus Calgar comics. Long story short, that thing is some hokey shit that fails to deliver since you know, why the fuck aren't you chucklefucks slaughtering in the battlefield for good Khorne?
In Age of Sigmar, the Skull Altar is available to your Khorne army for no matched play points and lets your Slaughterpriests re-roll prayer and judgement rolls when they’re nearby! This allows you to ensure a key Blood Blessing goes off and that you can deliver the Wrath-Axe to your enemy’s face with greater reliability. It’ll also further disrupt enemy Wizards, so if you pair it with the Hexgorger Skulls, it’ll be incredibly hard for sorcerers of all stripes to even use the most meagre of spells against you.