What are these things?
When I get close to it I take damage, but I can’t hurt it.
What does it do and what is it?
They are Demon Altars which are not monsters – hence you cannot get the tooltip by hovering over them.
You use them to craft some ‘boss’ summoning materials Slime Crown, Suspicious Looking Eye and Worm Food, as well as creating the stronger pre-hard mode weapon Night’s Edge.
Demon Altars can only be destroyed with Pwnhammer or Hamdrax after Wall of Flesh is destroyed. Trying to destroy Demonic Altar before defeating Wall of Flesh, you will take half your current health in damage minus armor, no matter the tool. Pwnhammer is acquired by defeating Wall of Flesh.
Destroying a Demonic Altar will “bless” your world with decreasing amounts of ore, starting with Cobalt, then Mythril, then Adantium and then starting the loop again. This means that it will convert random blocks into whatever ore it is blessing your world with.
It will also place Ebonstone/Pearlstone in random locations underground, allowing Corruption/Hallow to spread faster and from multiple locations.
Source: Terraria Wiki
Source : Link , Question Author : Sir Ksilem , Answer Author : Sadly Not