Skeletron is a boss in Terraria. He is summoned by talking to the old man at night and clicking "curse". He has 4400 health in total.
Skeletron attacks by trying to slap you with his hands. With the right timing, they are easy to avoid. He will also spin his head at you sometimes. While he is doing this, his defense drops to zero and he doesn't swipe his hands.
If he is not defeated before daytime, Skeletron will do his spin attack, but he will only take 1 or 2 damage, and he instantly kills you if you touch the skull.
Upon death, Skeletron always drops 5-15 lesser healing potions and 5 gold coins. He also has a chance to drop one or more of the following:
Book of Skulls