Faded altar
Version 0.17: This article may not be up to date for the latest stable release of Crawl.
Praying at a faded altar allows the player to convert to a randomly selected god. There is no way to know which god the altar will select for you, but it will be one your character is normally eligible to worship. Selecting a god at random is far from optimal, but doing so via a faded altar rewards you with a bonus +20 piety. After use, the altar will become a normal altar of the given god, allowing the player to perform any related altar-based actions (gold donations, weapon blessing, etc.).
If you already worship a god, using a faded altar will most likely result in you angering your former god when you convert. However, the conversion allowances offered by the good gods still apply as normal, and it is possible (though unlikely) that the randomly selected god will be the one you currently worship, in which case the only effect of your actions is the immediate gain of 20 piety.
History
Faded altars were added in 0.17. They were called "ecumenical" altars during development.