The Emperor never did anything, Humans where thousands of years away from existing when the war with the C'Tan was tearing up the galaxy.
For that matter Slannesh did not exist either.
The Outsider was tricked in to killing other C'tan because he was stronger then the rest, how ever the Nightbringer was most FEARED of all the C'tan because he controlled the largest army and traveled around tormenting all killing everything with no general plan in mind.
It is now established canon, the emperor whooped Voidy and put him on mars.
Voidy was hiding from the others on Earth at the time, eating maiden and slaying knights in the pre dark ages.
You're quite right Air meister he did fight a wyrm, and drowned it in Magma. Was thinking more about Vulkan for some reason, due to it saying something in the fluff about the emperor laying his salamander across a pool of lava.
Vulkan killoed a salamander in a contest with the Emperor. Ferrus killed a wyrm. They are both pretty similar characters; masters of forgery, kileld fire monsters, big hammers etc etc.
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