This dude was inspired by the representation of the Alpha Legion in this piece of artwork by Nicolas R Giacondino also known as Aerion the Faithful. I really liked this idea of the Alpha Legionnaires being almost faceless and if I had the money, I would build a whole force of these guys, each of them using the same parts. For now I'll just have to settle with this single Alpha Legionnaire. Out of all of them this model is the only one that doesn't have an alternate head, to further emphasise the secretive nature of the Legion.

Alpha Legionary, “Zero”
Not much is known
about this shadowy member of the infamous XX Legion during or before the Heresy
and those sparse records that have managed to survive through data loss,
censorship by the Inquisition or corruption over the course of the millennia
cites a particular Legionary that has been mentioned in several after action
reports and Remembrancers notes. These references are often counted as being
conflicted or inaccurate, as it makes mention of his presence in numerous
conflict zones over the course of the Heresy, from Istvaan III, Calth, Armatura,
even right up to the final Seige of Terra. This has led many scholars to
theorize that this single legionary may in fact be many, as there have been
noted instances where he was present in more than one place at the same time if
the date stamps of the records are to be believed. Alas these have been
endlessly debated by Archivists new and old and there is little hope the truth of the matter will ever be verified.
What is known is that in all instances of his mention, he was often been seen
observing the forces involved on both sides of the conflict, engaging only when
actively threatened and never removing his helmet to reveal his face, further
adding speculation as to his true identity. When asked for identification, the
legionary was noted to speak only the following:
“I am no one and
everyone. The beginning and the end. I am Alpharius”
From The
faces of Treachery by Darius O’Loren, redacted by the Inquisition M32.