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Under The Mask:

The skull gets a supply of blood to stay strong and healthy just like how bones work on humans. What happens if it's removed? Well it bleeds out, from where exactly? The eyes.

Older Generation Dranixions could survive without the skull since they have never developed one in the first place but they are fragile and have no way of defense compared to the newer Dranixions where they have that skull. These skulls were developed as natural selection in order for the specimens to survive a lot longer. They are quite hard and practically give immunity to headshots.

Effects:
-They have 1 hour to put it back on but the closer they are to the hour the more emotionally unstable they become until death arrives..
-There is no physical pain involved only emotional

Extra:
-Their eyes are just white and not their original colors you would see when they have it on
-You can see the old marking on their forehead, shapes may vary (DREX and DRAX are the only ones with a star marking)
-There is a way to stop the bleeding temporarily with a pill but it can't be taken constantly