Cassander's recovery is one stilted, not even the most skilled clergy are able to improve his health. The reason being is the nature of the heart that beats in his chest, for his original one had been torn out and replaced.
Yinan's demon heart has served him plenty, beating strong even when the rest of his body threatens to crumble and fail. But its power is limited by Cassander's very human shell. There's only so much demonic power it can withstand before his mortal body begins to fail.
