The creature is commonly known as "Death" or the Grim Reaper. It is not known whether Death is a singular creature (that rejuvenates after being destroyed and can copy itself) or a race of creatures.
In game terms, the 3.5 monster manual has stats under Wraith, Dread.
In terms of what you know about the creature, your best guess is that one hit is sufficient to kill most people and anybody that is hit by it more than once is definitely dead.
More Info that Knowledge Religion and Knowledge Planes reveals about "Death."
Death … Once it selects a target, it does not stop until the target is dead or the death is destroyed. A target may flee, but Death is relentless. Death is rarely fooled by illusions.
When others intervene and attempt to prevent the Death from pursuing a target, Death might copy itself and create more Deaths to slay the interveners.
Death's Scythe never misses.
Death is not known to use transportation magic (teleport, gate, etc.), but it can be summoned.
A powerful spell caster can keep Death at bay and/or dare to command Death, but Death always has the last laugh.
Combat on the Shadow Plane
The only way off the plane is for Quertus to take the party with him. If you seperate from Quertus (or if Quertus dies), the other characters have no known way of getting off the Shadow Plane (i.e. you can wander around until you starve or are killed or are lucky enough to find a portal).
Movement is enhanced: A 5' step is equal to 250'. A 30' move is equal to 1500'. Flight and other movement rates are also enhanced (by x50).
Shadow Spells are enhanced, as are Shadow/Negative Energy creatures.
Light and Fire spells are reduced in effectiveness.
Quertus can take the party off the plane by focusing (standard action ending the Shadow Walk spell, but the party members have to be close by.)