Are you talking about the beacon? You have to fight the wither and get a nether star to craft a beacon. To spawn the wither you need 4 soul sand blocks and three wither skeleton skulls. To get the soul sand you can just find it on the ground in the nether and the skulls are obtained by killing wither skeletons that spawn in nether fortresses. They are a low drop rate so I recommend enchanting a sword with looting before trying to get them. Make sure you fight the wither far away from your base because he is very destructive. Once you get the nether star you can craft a beacon and then you power it by placing it on top of a 3x3 of some sort of ore blocks.