What is the most explicit description of the Unlife? Was there any? Has anyone thought of a cool way to explain it in a little more concrete detail?
I believe the earliest description was in the Loremaster module
The Iron Wind.
I always saw the Unlife as something akin to a natural constant or a negative energy that exists beyond even the Void, and from which all darkness and negative energy flow. Similar concepts might be The Nameless One from the works of Raymond E. Feist, The Negative Plane from D&D, The Abyss from
Andromeda might also fit. I don't see it as a being per se, but more like a negative power that influences & corrupts other beings, and then works through them to spread. It's never really been clear to me if it even has a consciousness, and it may just exist as a corrupting influence.
I think one thing that is clear is that you can't fight the Unlife directly; the PC's can only fight its agents. Even when you win, it's only a matter of time before more fall under it's influence.
I would start with the description in
The Iron Wind though.