Needs review
Source: Complete Champion

Description: When preparing your spells for the day, you can leave your domain slots open. You can then choose, when the situation arises, to cast any of the domain spells you could normally have prepared for a given level. However, you are still limited to one domain spell per level per day.

Prerequisites: Ability to cast 3rd level spells, Access to two or more cleric domains

You may spontaneously convert any prepared cleric spell (except a domain spell) into a domain spell of the same level in the selected domain, just as a cleric channels energy to convert spells into cure spells.