Inexplicable twist of fate? No way - you have your reason for their power boost right there!
***
Two kobolds - the last two survivors of a bloody fight to the death - enter their warren. Bodies surround them, bodies burnt like the damned in Hell. Soldiers. Women. Hatchlings.
One of the kobolds falls to his knees and wails uncontrollably, horrible, torturous wails that echo down the cavern. It sends the vermin who had come to feast on this grusome banquet scurrying away.
The other kobold searches the charred remains and finds a twisted bone ring. He pries it off the body's ring finger. Burnt flesh sloughs off. He squeezes it in his scaled hand until it draws blood.
'I have entered the Endless Night, oh my Queen. You say "Forgive no wrong and let no slight go unpunished." I ask you for the power to right this wrong, to punish those who did this to me. I pay you with the only thing I have left in this world - my soul, and the souls of those who did this.'
***
Now - make up some new monsters that play off of that. 2 Level 5 Elites. They both have that ability that I created above.
One of them has something like this:

Wail of Despair (standard; recharge

)
Close blast 3; +10 vs. Will; 3d6+4 psychic damage, and the target is
dazed (save ends).
Not Again (immediate reaction; when first bloodied; encounter)
The kobold's Wail of Despair recharges.
The other guy has some funky stuff, too.