The Living Corpse Insect. The Living Corpse Insect is a insect that when eaten, enhances the users power dramaticaly. Although when you die the Insect takes control of your body, and attacks whatever is alive. Although the insect has no emotions, thoughts or memories, it has enormous power and attacks on instinct after the host has "kicked the bucket" or died.
Since there are no known users we can´t really say we know the full extent of the "power boost", or if there are no "side-effects" when they are alive. Since it has been said that there are no side effects by Yao Lao, we can expect that there are none. But we cannot "know", unless we truly see a living person with the Insect inside of them. Since according to me this insect seems to good to be true, there needs to be some sort of side effect except the one about the "walking dead" thing.
The side effects that I can think of that would reasonable would be: Enhanced Metabolisim, your life is shortened, your telomeres degenerate faster, might be hard to control the sudden boost in power. But these are all speculations.
Hope I helped.