The breath weapon of a creature with this feat forms a lingering cloud.
Prerequisites: Con 15, breath weapon with recharge time expressed in rounds, Clinging Breath.
Benefit: When the creature uses its breath weapon, it can choose for the effect to remain for 1 round as a lingering cloud of the same shape and size as the original breath weapon.
Anyone who enters the cloud takes one-half of the breath weapon’s normal effects; any saving throw the breath weapon normally allows still applies. Damaging breath weapons deal one-half their normal damage, and breath weapons with effects that have durations last for one-half the normal time. If a creature is affected by the same nondamaging breath weapon twice, the effects do not stack (use only the longer duration).
Any creature in the area of the original breath weapon takes no additional effect from the cloud, provided it is outside the cloud by the end of its next turn.
When a creature uses this feat, add 2 to the number of rounds it must wait before using its breath weapon again.
Source: Monster Manual IV, page 203. This feat originally appeared in Draconomicon (page 71); this is a revised version.