Breath of Siberys( Dragonmarked, p. 135)
Your breath weapon becomes even more deadly.
any true dragonmark, Breath weapon,
Your breath weapon deals an extra 1d6 points of damage, or an extra 2d6 points of damage if you bear a Siberys dragonmark.
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).