And a new domain to go with it.
Necropolis Domain
Granted Power: Disease resistance: +4 divine bonus on saves against diseases.
1 Deathwatch. Sees how wounded subjects within 30' are.
2 Gentle Repose. Perserves one corpse.
3 Negative Energy Protection. Subject resists level and ability drain.
4 Death Ward. Grants immunity to death spells and effects.
5 Hallow/Unhallow. Designates locations as either holy or unholy depending on cleric wielding positive or negative energy.
6 Forbiddance. Denies area to creatures of another alignment.
7 Repulsion. Creatures can't approach you.
8 Create Greater Undead. Mummies only.
9 Soul Bind. Traps newly dead soul to prevent resurrection.
If you need information on the Necropolis Litharge, I can recommend the Sandman Comics by Neil Gaiman.
And speaking about the breath weapon, IMHO, maybe you should give it a level drain attack as well. That would work real good with a line of negative energy.
I doubt it needs a Level Drain attack, but if you feel like it you can allways give it Enervation as a Mature Adult and energy drain as a Great Wyrm, through it's sorcererous abilities.
I won't prevent you form creating one, but remember that vile damage is a lot harder to heal than normal damage. Now a breathweapon that deals corrupted damage, that's something...
By the way, the Obsidian Linnorms breahtweapon is a cone of petrification It will also get a few very nasty spell-like abilities. (Like one that transforms stone to sand... think about it... )