[As of now, especially with the healing disabled, this build is nowhere near viable]
The cornerstone of this build is the interaction of Nusemnee's Atonement and Lightning Soul.
Here are the wordings:
You gain a +2 feat bonus to saving throws against charm effects.
Whenever an attack you make would damage an ally, you can choose to take the damage instead. You have resist 5 to damage taken this way. You can choose to take the damage even when you are dominated. This resistance increases to 10 at 11th level and 15 at 21st level.
You gain a +3 feat bonus to lightning damage rolls. This bonus increases to +4 at 21st level.
In addition, whenever you take lightning damage from an attack, one enemy within 5 squares of you takes 10 lightning damage. This damage increases to 15 at 21st level.
I can't find any solid rulings on the interaction, so I guess it depends on the DM.
If you or your DM do not agree that Atonement triggers Lightning Soul, this build is useless, you can probably stop reading now.
But as far as I can see, it should work by RAW.
Dragonbreath attack targets ally, you choose to take the damage (source is still the attack), you gain resistance to the taken damage (DB would pierce your own resistance otherwise).
As stated above, this build is not viable due to lack efficiency, especially since the healing doesn't work.
This particular build is retired indefinitely, until I can overhaul it.
On to the build:
====== Created Using Wizards of the Coast D&D Character Builder ======
Dragonborn, Sorcerer|Cleric, Breach Warden
Fey Beast Tamer Starting Feature: Fey Beast Tamer Companion (Young Owlbear)
Sorcerous Power: Sorcerous Power Dexterity
Hybrid Cleric: Battle Cleric's Lore
Hybrid Talent: Soul of the Sorcerer
Soul of the Sorcerer: Dragon Soul
Dragon Soul: Dragon Soul Lightning
Dragonborn Racial Power: Dragon Breath
Dragon Breath Key Ability: Dragon Breath Constitution
Dragon Breath Damage Type: Dragon Breath Lightning
FINAL ABILITY SCORES
Str 14, Con 24, Dex 12, Int 11, Wis 11, Cha 16.
STARTING ABILITY SCORES
Str 13, Con 18, Dex 11, Int 10, Wis 10, Cha 10.
AC: 34 Fort: 29 Reflex: 23 Will: 27
HP: 111 Surges: 13 Surge Value: 34
Diplomacy +16, Endurance +20, Intimidate +18, Perception +13
Acrobatics +9, Arcana +8, Bluff +11, Dungeoneering +8, Heal +8, History +10, Insight +8, Nature +8, Religion +8, Stealth +9, Streetwise +11, Thievery +9, Athletics +10
Level 1: Hybrid Talent
Level 2: Ancient Soul
Level 4: Nusemnee's Atonement
Level 6: Draconic Spellcaster
Level 8: Daunting Breath
Level 10: Defender of the Wild
Level 11: Lightning Soul
Level 12: Enlarged Dragon Breath
Level 14: Radiant Breath
Level 16: Thundering Breath
Hybrid daily 1: Beacon of Hope
Hybrid utility 10: Steeled Against It
Arkhosian Scepter Accurate rod +4, Solar Wyrmscale Armor +4, Healer's Brooch +4
I left out a lot of items and powers from the build, because I haven't finalized what's best for it.
Only items/powers I've confirmed as useful are included.
I ran out of slots for Resounding Thunder, consider it the next feat for lvl18 (or Oncoming Storm).
Some explanations on the build choices:
1) Why Hybrid Cleric?
a) Because a pure Sorcerer would boost the DB damage further with Draconic power with Str, which we need for the PP.
This is bad, because we want to minimize actual Dragonbreath damage so we don't kill ourselves.
b) Because it gives you access to 2 defensive features/feats: Battle Cleric's Lore and Radiant Breath.
Battle Cleric's Lore is just great as a hybrid pick-me-up: Scale proficiency and +2 shield bonus.
Radiant Breath allows us to take radiant self-damage, which will trigger Solar Armor, allowing us to use a surge as IR (so only once per turn).
Without this option, this build will kill itself pretty fast.
Update: IR can't be taken on a player's own turn, stupid of me. Will have to find a better way to heal/mitigate.
2) Why Fey Beast Tamer?
Because the more allies hit by Dragonbreath, the better. The theme allows for a free ally, and I picked Young Owlbear. This is because it has the lowest Reflex score out of the choices, the only one that allows us to hit it on a roll of 2. A damage boost against enemies (only) also helps.
3) Why Breach Warden?
Why take the trouble of MCing Warden, when the Str requirement can boost DB damage (which is bad for us)?
Because of the Elemental Breach feature:
After each of your extended rests, choose a damage type from the following: acid, cold, fire, lightning, or thunder. The choice remains until you take your next extended rest, at which point you can choose the same damage type or replace it with another.
Whenever you take damage of the type you chose or you score a critical hit using a warden attack power, each enemy marked by you takes damage of the chosen type equal to your Constitution modifier.
The mark will come from the Daunting Breath feat, just be sure to attack the enemy target FIRST.
Here's a summary of what happens if you use Dragonbreath (at level 16, using listed build):
Assuming you are within 5sq of the enemy target and your Fey Beast companion is within DB blast range:
Dragonbreath +25 vs. Reflex (as minor)
2d6+14 lightning, radiant, thunder damage (ave. 21 dmg to enemy, Young Owlbear +2 dmg roll aura is not included in calculations)
You mark the enemy.
You take 6-12 damage through the Young Owlbear, allowing surge usage, marking all allies hit (yes, they get -2 to attack rolls that don't include you).
You zap the enemy for 17 damage (Lightning soul 10 + Elemental Breach 7)
So it's an average of 38 damage to the enemy, and 9 damage to yourself.
For each additional ally hit, add another 17 damage to the enemy, and 9 damage to yourself.
Assuming 4 allies hit (3 PCs + Young Owlbear): 89 enemy damage, 36 self damage.
Assuming 7 allies hit, because your party went FULL OWLBEARS (3 PCs + 4 Young Owlbears): 140 enemy damage, 63 self-damage.
All this for 1 minor, which recharges itself (Ancient Soul), so you can blast 3 times a turn.
But the central point to all DB builds is: is the damage to the enemy worth the self-damage I'm taking? NO, NOWHERE IT YET.
The surge/turn granted by Solar armor mitigates the self-damage somewhat, healing 37 HP per use (Healer's Brooch), 42 with Beacon of Hope.
=HEAL ON IR DOES NOT WORK=
So yes, if you limit your targets (as if 7 allies hit/breath is a likely scenario), you can easily heal or at least minimize all self-damage done.
Using Steeled Against it (Endurance daily 10) reduces self damage by 5 (it stacks with Nusemnee's Atonement resistance).
But there are still drawbacks:
a) Your build revolves around Dragonbreath, making your other attack powers mediocre and inaccurate
b) You're marking your allies. -2 penalty to hit is never, ever a good thing.
The first can be mitigated by choosing powers that have an Effect line, so you'll get the benefit whether you hit or not.
The second is there to stay, unless you choose an alternative Paragon Path.
Frankly, I haven't explored all PPs, so if anyone can suggest alternate ones, go ahead.
Fiery Blood (+Arcane Admixture) is a possible expansion, but I don't like it because it only damages adjacent enemies (for only 5 dmg), which severely limits your Close Blast targetting. Using Hurl Breath instead often involves you hitting yourself, which is not good in my book (you pierce your own resists).
Oncoming Storm is good, as your Dragonbreath becomes more accurate as it hits more targets, but needs Thundering Breath.
Weaknesses: Resist lightning will kill this build pretty bad as it depends on multiple plinks of lightning damage. Consider investing in Gloves of Piercing.