This is a nice idea, my main concern is that it is underpowered. The closest analogues are Blessing and Chaos Power (upgraded). Blessing is 1 more quanta for the same effect without the downside, and with a bonus to HP. Upgraded Chaos Power gives an average of 3 Atk, again with a bonus to HP instead of a downside. I'll go with blessing for the calculation because it isn't random and thus more definitive.
Common Attributes
1 
Cost.
+3 Atk
Burning Nimbus
Negative
Deals 2/3rds of total Atk damage to owner.
Improved Blessing
Positive
+3 Defense
Negative
+ 1 
casting cost
So basically in using this over Improved Blessing, you are trading a 1 lower quanta cost for 2/3rds self-damage and not getting +3 defense.
Terribly inefficient damage-wise, but I could see something like a fire-time scorpion deck based on it and using the remaining Nimbus as CC, potentially with SoSa. But unless you want CC to be the primary function of this card while avoiding damage somehow (whether it be SoSa, Phase Shield, Delaying/Freezing the creature, or otherwise), and want it to be a smart option to use on your own creatures, I would recommend a change of some sort.
I think this is correct. I didn't realize it before, but It does overlap with those cards, costs pretty much the same and has less benefits. Maybe +5 damage to opponent and 1/3 self damage (like choongmyoung said) could make this card a viable option.
I still consider it a good idea though. I mean, it can be used in a weapon!! that's cool!
There is a key difference here, think of what happens if you use this with say adrenaline on opponent creature then put up a phase shield, sundial, etc.
Its a double edged sword, so it cuts both ways.
Consider for instance hitting the enemy's crimson dragon with this + BB.
