Deep within lurks, stealthy, shifty, a creature of intangible proportions, its form indeterminable. It thrives in the darkness and has no desire for the light, secure in the knowledge that its will... no, its orders, will be carried out, or at least seriously considered. In time, it knows, its propositions will prove irresistible.
It revels in this, its suggestive power. And while it knows that it's effectively, and irreparably, destroying its host and, by extension, itself, it's unworried. In fact, it experiences a certain carnal pleasure in the havoc it wreaks on its incognizant host.
This is what it was created to do, and its singleness of purpose is absolute - to consume, to pillage, to deteriorate and eventually destroy. It will see its surreptitious occupation to the end, till its host's demise.
Meanwhile, oblivious of this insidious plot, the host wanders about, day in day out, unaware of the ominous presence within.
Whilst doing the creature's bidding, the host is only mildly aware that the indulgences are increasing in intensity, in fervor, in frequency, but is powerless to resist. It's nothing to worry about, the host rationalizes, unaware that this thought too came from the darkness within.