View source for Altar of the Old Ones (3.5e Location)

=== Description ===

Altars of the Old Ones are usually associated with a single entity, and designed accordingly.  The altar of Cthulhu may be underwater, wrapped in basalt stone tendrils and strange non-euclidean geometry that baffles the mind.  Meanwhile an altar to plutonic death god may lie underground hidden from view and surrounded by the skulls of the dead piled high into great supporting pillars.  All these places have in common is an altar or platform from which the rituals are to be observed, and conditions required for their awakening and transformation.  When the proper actions are fulfilled the area seems to transform and come to life with hidden magic, alien growths, or shifts in apparent time and space.  For those which call elder evils, these altars and their immediately surroundings are subject to a localized and increasingly powerful elder sign which comes to a head when the attention of the elder evil bears upon them.  Fortunately for the world, these phenomenon do not spread far.  However, it may result in consequences depending on what fell deal has transacted.

Rarely these places become associated with gods or archfiends.  The results are similar, though they may be enshrined in places more public and close by if worship of these entities are commonplace.