Alyanna territory was deep within an expanse connected to Zala dimension's soul realm. Zala had given the Alyanna control of this area long ago, even before Trannisa had to take them in. It was a special area within the soul domain that fed the soul. There was always a radiance that gave any particular soul exactly what it needed to be whole or strong.
Being given this area essentially made the Alyanna the soul keepers of Zala dimension. This is why their death domain is also unique and important to Zala. Both of these areas house souls attached to Zala and souls for the exclusive use of the Alyanna.
It had always been up to the Alyanna not to abuse their unique situation.