where they come from
Altars are placed when the world is generated. You cannot craft one, mine one, or carry one around. Most of them cluster in and around the chasms of your world's evil biome, with a handful scattered through the ordinary underground, and the game only makes a limited number of attempts at those strays, so a world can end up with none outside the chasms at all. They give off a faint glow, which is usually how you notice one in an unlit cave. If a world somehow ends up with none left standing, the Eye of Cthulhu starts dropping a placeable altar to cover the hole.
keep one near your base
This is a station you come back to repeatedly, so it is worth finding the closest one and building a room around it instead of making the trip every time. There is very often an altar not far below the surface near spawn, so start looking around zero depth before you dig deep. What it makes is the point: the pre-Hardmode boss summons, the Night's Edge, the Void Bag and its Vault, and the Deer Thing. That is also the argument against demolishing every altar you own later on.
do not swing at one early
Hitting an altar with a hammer before the Wall of Flesh is dead does nothing to the altar and takes half your health. There is no trick hiding behind it. The one exception is the rare case where two altars generated stacked together, in which case a pre-Hardmode hit destroys both, which is the only time anyone does it on purpose.
breaking them in hardmode
After the Wall of Flesh falls, a strong enough hammer, which in practice means the Pwnhammer or something better, will break them. The first break decides between Cobalt Ore and Palladium Ore for the rest of your world's life, the second settles the mythril tier and the third the adamantite tier. None of it is decided until the swing lands, so saving beforehand and reloading is a legitimate way to steer which half of a pair you get.
Keep going and the cycle restarts, but every pass seeds less than the one before: the second run yields half, the third a third, and so on downward. The total is unbounded, it just climbs slowly, and no amount of breaking will ever introduce the ore your world did not roll. Rarity stays fixed too, with the cobalt tier the most plentiful and the adamantite tier the scarcest. The new ore appears across the whole underground down toward the Underworld, in caves you have already cleared as much as in ones you have not.
the cost of each break
Every altar you destroy releases a Wraith or two regardless of the time of day, and they hit hard for someone who has only just entered Hardmode. Clear the last batch before breaking the next one or you will end up fighting a crowd of them in a tunnel. Breaking altars also switches on the nightly chance of mechanical bosses arriving unannounced, and opens the door to random pirate invasions, so the world gets noisier the moment you start.
practical notes
- Cut routes to several altars before you kill the Wall of Flesh. Hardmode corruption and crimson are far nastier than the versions you have been walking through, and having the tunnels already dug saves doing it under fire.
- Block Swap works on the blocks beneath an altar even before Hardmode, which is handy for turning a chasm floor into platforms for an arena. On ebonstone or crimstone it needs a Nightmare Pickaxe or Deathbringer Pickaxe, or a dusting of Purification Powder first to make the stone ordinary.
- An altar sitting on sand will not fall if you mine the sand out. The altar hangs there and the sand above it stays suspended.
- Meteorites will never land on one, so an altar in the open does not block your sky.
- Smashing is optional. Fished crates supply every Hardmode ore, both halves of each pair included, so a single world can get everything with every altar left intact.
the purple spiky thing
New players reliably fail to work out what these are, to the point that asking about the purple spiked thing turned into a running community joke, and one of the game's own title screen messages quotes it back at them.