Can a mighty Nightlord Terminator go back in the past, turn loyalist and become a pre-heresy Deathguard? Yes, he can:
At the beginning this miniature had to become an Adeptus Titanicus Knight, then a truescale Crimson Fist, but I've always wished a Horus Heresy Deathguard in my collection.
So, here you are my new conversion/renovation:
Tallarn Highlanders refer to him as
Indomitus (they misunderstand his armour's name):
Before the paint:
One of the initial plans: