Ok the references for this are in the first couple Horus Heresy books and the Ultramarine book "Courage & Honor"

I had a thought about if the same weapon that struck down Horus was the same one used to strike down the demon M'kar

Here is my reasoning
-It was stolen from the Interex by Eberus
-Could kill a person if you knew their true name
-Called Shard of Eberus by M'kar and he feared it
-Uriel ventris knew M'kar's true name and it was used to kill/banish the deamon