Altars are the base blocks for all of the different Altars. They are decently hard to acquire, needing Altar Fragments (a rare drop from Fossil Ore and dungeon chests) and Obsidian. On its own, it's worthless, but in a combination with the right item, it can be a great help.
There is also another form of altar necessary to reach the Erebus; it is called the Offering Altar, and it is crafted differently (as it does not need Altar Fragments).
Right-clicking an Altar with the correct item will make it consume the item; you then need to use a Wand of Animation on it to fullfill the Altar Infusion.
The Offering Altar Edit
This altar is set apart from the rest, due to its wildly different purpose. You will only likely need it twice; to create two Gaean Gems for two Staffs of Gaea, one to get to the Erebus and one to get back.
It is crafted using Smooth Stone, Obsidian, Gold and Stone Bricks, as seen below. The corners can be any variant of Smooth Stone, including Umberstone, but the three cardinal directions (East, South and West) must be Stone Bricks.
To place the necessary Emerald, Diamond and Obsidian on the Offering Altar, right-click it with the item held. Once the Gaean Gem is created, you'll have to shift+right click the altar in order to retrieve the Gaean Gem. Alternatively, you can break the altar with a pickaxe, which will drop both the altar and the gem.
The Different AltarsEdit
There are currently 4 different Altars, but expect more to be added later on. They all have special abilities that are very useful. Altars will, however, lose their effect. Some are bound to time (such as the Altar of Healing), but the Altar of Repair reverts back to a normal Altar after one repair. Remember; you need a Wand of Animation to properly infuse an Altar.
Altar of HealingEdit
The Altar of Healing heals all players within a 4 block radius completely, making it a possible lifesaver and turning your home into a true safe haven.
Infuse the Altar with Bioluminescence to get this effect.
Altar of LightningEdit
The Altar of Lightning shoots lightning at arthropods, making it useful for mob farms and protecting your home.
Offering a Redgem on an Altar will give the Altar this ability.
Altar of RepairEdit
Repairs your tools and armor fully, even the enchanted ones.
Elastic Fibre should do the job for this one.
In order to use this Altar properly, stand right next to the Altar, as close to its center as you can get, then throw the item to repair onto the magical anvil on top. Step back, allow the Altar of Repair to work for around five seconds, then you'll hear a hammering noise and observe the Altar revert. Your item can now be collected and is fully repaired. This system also works on Wands of Animation, making the process effectively infinite, depending on how much Elastic Fibre you wish to grab.
Altar of ExperienceEdit
This Altar allows you to transform various Erebus-related mob drops into pure experience.
Bio-Velocity is the thing you need.