Another use of altars is sacrificing.
Altars also create the domain of the gods. You know when you're in such a domain when you get the message "You feel the presence of <god>". When you first build an altar in an area without a domain, you will receive the message: "The domain of <God> now has reached this place."
The Domain with the greatest "power" takes effect, and only 1 domain can be in an area.
The "power" and size of a domain is controlled by the quality of the altar, for example a 10ql golden altar will have 18 tiles radius, 50ql - 57 tiles, 90ql - about 96 tiles, a stone altar of 21.97ql has 24 tiles radius. The domain seems to get weaker the further it is from the altar and might get overpowered by another god's domain.
It should also be noted that a few altars are still "bugged" as they were made before the recent domain changes. (5 June 2012 - faith-zone-changes-achievement-bugfix).
Domains get updated approximately 5 minutes after a server is restarted, so for a small period of time after a restart, there are no domains. A new altar's domain can take around 5 minutes to update, and a domain change can take up to 10 minutes.
When you are in the domain of your God and have at least 10 faith, you'll get certain bonuses, including a fighting bonus. The amount of the bonus is affected by a great number of things, such as faith, alignment, the type of the altar (see below), the distance to the altar, the quality of the altar, and if you're a Champion.
Priests also get several bonuses from altars, including a bonus of up to 20% to successfully cast a spell, and an increase to the chance of casting a higher strength enchant spell.
You can have 1 domain below ground and 1 other above ground without them affecting each other.
Underground altars will still give their bonuses above ground. For example, underground Libila altars will spawn mycelium above ground. However, you will not receive the domain message moving through these underground altars.
If you are trying to make an altar for a Priest of a different religion, i.e. you are a black lighter and your priest is Vynora, you can build the altar, and have your priest put the last piece of the altar on by unclicking faithful and attaching the last piece. Works for wooden and stone altars.
Destroying an altar
If you desecrate an altar of your deity you stop following that god and have your faith and favor reset to 0. Exorcism xp is gained. The altar is removed from the game.
Before desecrating, it should be noted that mayors and everyone with 'destroy' permission are able to equip a maul and have a minimum of 10 faith. Right click an altar to select the destroy (or desecrate) option.
Do not desecrate your deity's altar unless you are certain you want to deal with the faith and favor resets. There are no known penalties for destroying an altar on deed if you are the mayor.
Types of altars
There are four types of altars. Wooden altars are the worst type, gold altars are the best type. The metal altars can be created inside each others domain, while the stone and wooden altars cannot. However, you can move a finished stone altar into another domain. Domains do not cross the cave/above-ground layers, so you can have one domain underground and another on the surface. this can be an easy way to get several types of altars in one spot - create them below ground and push them above as you make them (or vice-versa).
Metal altars seem to give gifts and potion more frequently.
To find a local Altar:
Hit 'escape' key to open up the menu , Click on spell effects.
The power of the faith bonus will go up as you closer to the altar.
Note it goes to the strongest one at all times.
Far from the altar is 12 and at it is 55.
The type, god , type and Quality of the altar does not change these values.