I've been reading the Dark Angel's parts of the Horus Heresy recently and I'm trying to understand the Lion's motives.
My understanding is that
Caliban is rotten because of a demonic presence called the Ouroboros in the heart of the planet.
The Lion is aware of this and
Antagonises the Knights of Lupus into war because he believes they are researching The Ouroboros and The Great Beasts
When the Emperor arrives and leaves to take part in the Great Crusade he quickly sends Luthor and Zahariel back to Caliban because
He wants to limit the "taint" of Caliban to the world and not let the secrets they don't realise they know spread out into the expedition.
However he also appoints the
Last Knight of Lupus
To be the new Lord Cypher because
As a knight not from the original Order
his loyalty would be to the Lion, not Luthor or Zahariel.
However, this doens't make sense to me. Surely
As The Lord Cypher is a member of the Knights of Lupus the man he's using to keep an eye on Luthor and Zahariel is actually the most untrustworthy of all of them?
Why does the Lion make this appointment?