Hallowed Altars are generated at worldgen, and sometimes will spawn in low quantities. Therefore there is a crafting recipe for this tile. It is 50 Pearlstone Block, 10 Hallowstone Blocks, 5 Hallowed Bars, 20 Souls of Plight, 30 Purification Powder, and 15 Light Shards. The most vital material in the crafting of this tile is the Hallowstone Block, which only spawns in the Sky Fortress. To smash a Hallowed Altar, the player must use the Demon Hammer or a better hammer. When you break one, there is a 10% chance for a Comet to spawn, and every three smashes, Magmatic Ore will be spawned in your world, and 1-2 Armored Wraiths will spawn near the player.
In the source mod, Hallowed Altars are also created at worldgen and will most commonly appear at altitudes from the surface down to the underground layer. Like in the tConfig version, Hallowed Altars are relatively rare, so they have a crafting recipe: 10 Hallowed Bar, 20 Pearlstone Block, 20 Pixie Dust, 2 Light Shards, 2 Sacred Shards, and 1 Soul of Blight.
They can only be smashed in Superhardmode with The Banhammer or better. Upon smashing the first altar, either Tritanorium Ore OR Pyroscoric Ore generates in the cavern layer. The second altar being smashed causes either Unvolandite OR Vorazylcum to be spawned.
Magmatic Ore: An ore that generates on every third smash of a Hallowed altar. Can be crafted into a Magmatic Long Sword.
Ever Ice: Obtained from Comets that fall on Hallowed Altar smashes.
Source Mod Version
Tritanorium/Pyroscoric Ore: An ore that generates on the first and every odd smash. Can be crafted into bullets, a sword, and a pickaxe. Both of these ores will hurt the player on contact. This damage can be prevented with the Tome of Luck.
Unvolandite/Vorazylcum Ore: Ore that generates on every even smash. Can be crafted into a sword, a picksaw, a gem staff and an armor set both involving the Kunzite gem.