Breath Of Siberys

Your breath weapon becomes even more deadly.
Prerequisites: Breath weapon, any true dragonmark.
Benefit: Your breath weapon deals an extra 1d6 points of damage, or an extra 2d6 points of damage if you bear a Siberys dragonmark.
Note: A breath weapon is available as a racial ability of the half-dragon (MM 146) and as a class feature of the dragon shaman (Player’s Handbook II 11).

Source: Dragonmarked