N'Zoth whispers: Six seats at the high table. Six mouths that hunger. One will consume all others.
N'Zoth whispers: It grows hungrier... bolder. Alas, your eyes are closed.
N'Zoth whispers: The Light has struck a bargain with the enemy of all.
The blind queen wields a scepter of bone. From the deep, she calls forth doom.
Queen Azshara yells: Treacherous Banshee! Do you think I am blind to the darkness you seek to unleash?
Why would the Void be afraid of Death?
It makes no sense. Why would a force that can consume all of reality be afraid of a force that can only affect living beings?
The Void is ancient as hell:
"In the very beginning of time, before even the cosmos existed, there was only the Light and the Void. Originally, only the Light existed as an unfettered, shifting ocean of living energy. However, as some of its energies faded and dimmed, pockets of cold nothingness were created. From the absence of Light in these spaces, a new power coalesced and came to be. This was the Void, a dark and vampiric force driven to devour all energy, to twist creation inward to feed upon itself. The Void quickly grew and began to move against the Light, and before long, the mounting tension between the two forces ignited a series of explosions that ruptured the very fabric of creation, giving birth to the physical universe."
It existed before the physical universe even existed, and was one of the two fundamental forces that are responsible for its creation. Death, on the other hand, was only created when the first mortal being appeared - which would make it a much younger force than the Void.
Scientifically speaking, Void is empty space. And empty space is not affected, whatsoever, by biological decomposing processes, such as Death. It would be like comparing Biology with Physics, and saying something like a plague would be worse than a black hole.
Speaking of black holes, the Void operates much like one. Its purpose is to consume all of reality, leaving nothing behind but empty space. Contrary to that, Death's goal is to, pretty much, kill every living being in the universe - which, would still leave us with existing energy and material. The physical universe would still exist, in the end of the day. It would just be emptier - without any living beings in it. That is not nearly as bad as reality being reduced to nothingness.
Warcraft 3 even solidified its supremacy over Death:
https://m.youtube.com/watch?v=e9I3jNzrTx8
The following is taken from the Legacy of the Damned campaign of Warcraft 3, Chapter 7, mission two - The Forgotten ones:
Anub'arak: We have entered the Old Kingdom. Be wary, death knight. Whatever scared the dwarves still lurks down here somewhere.
King Arthas: What the hell are they?
Anub'arak: They are the horrors the dwarves spoke of. I thought they were only legends.
King Arthas: What?
Anub'arak: They are the faceless ones...
Anub'arak: It cannot be...
Anub'arak: A forgotten one. Look to your defenses, death knight! Fight as you've never fought before!
Anub'arak: We cannot take them all! Hurry, death knight--up the stairwell! Hurry!
The exchange between the two clearly shows that Anub'arak and Arthas were afraid of Faceless ones and the Forgotten one. They couldn't even defeat them all.
Now, it is said that the Void Lords are so incredibly powerful that they can't even manifest themselves in the physical universe. That is why they had to, literally, thrust the Old Gods into our universe, for them to spread their influence. The Void Lords are so immensely powerful that a mere echo of their power - Dimensius the All-Devouring - managed to completely annihilate the Ethereal's homeworld of Ka'resh.
I'll have to admit, though, that this power is not just attributed to the accomplishment of a Void Lord. Murmur, the sound elemental, is said to be able to destroy entire worlds with a single whisper. However, this is not the case in Outland. This could be explained the same way Dimensius the All-Devouring is easily beatable in Outland: They have not fully emerged into our universe. I highly believe that the case of Murmur should be explored more thoroughly, as we haven't got the slightest idea of what a sound elemental is capable of, or even the very nature of it. (Bards could be hella OP as a future class )
The only reason i can see the Void "fearing" Death is due to their inability to manifest in the physical universe. What i mean is, the Void Lords planned on corrupting Azeroth to serve as their most powerful entity. Sargeras even believed that if they succeeded, that the whole universe would, pretty much, be doomed. Now, since Undead are believed to be resistant to Void corruption, it is possible that the force of Death (probably the Jailer) is trying to convert Azeroth into an Undead being - which, would foil the Void Lords' plans to corrupt her.
That is the only reason i can see them "fearing" the force of Death. Not much of a fear, to be exact, but more of a grudge towards it.