3/5 stars. The rules are solid.
Background.
These blackshields are likely to be original Great Crusade warriors. They are the elite veterans of the legions and likely long since forgotten by their parent legions. If it were not for the civil war, they probably would still be meandering around the outer reaches of the Galaxy. But now, they're bringing their convictions back to the centre.
Rules Evaluation.
No troops. They don't exist here. Instead, you get one compulsory elite, and 2 extra elites slots. This, of course, calls for an all terminator army to my mind!
But its better than that: you can pay 50 points to upgrade legion veteran squads to have Line, Heart, and Fury of the Legion.
So take your veterans, terminators, and some auxiliary units and you now have your elite of the elite of the great crusade coming back to haunt the galaxy. I could easily imagine something like pre-primarch legionaries for this oath.
The main issue I see here is which other oath (if any) to select. Many have narrative challenges to my mind.
Combinatoric Analysis.
The Eternal Vendetta: ⭐️⭐️. 2/5 stars. Appropriate and fluffy. But rules-wise is lacking in applicability.
Panoply of Old: ⭐️⭐️⭐️⭐️. 4/5 stars. For those of you who really want to go down the road of pre-primarch legions, this combination stands out. War Hounds instead of World Eaters is an obvious one to my mind!
Only in Death Does Duty End: ⭐️⭐️⭐️⭐️. 4/5 stars. The narrative is strong here, and I could genuinely give it 5/5 stars on a good day.
The Spoils of Victory: ⭐️. 1/5 stars. I really do not like the narrative here at all. It might work with some of the more, shall we say, evil legions elites (Night Lords?). I just can't quite picture it really as it is in direct contest with the ethos of this oath.
An Eternity of War: ⭐️. 1/5 stars. Elite veterans who have turned mad. Yet are prideful of their armour. Maybe they still think the Great Crusade is in full swing? Its sadly another combination that works with the rules, but for the narrative isn't great.
The Flesh Is Weak: ⭐️⭐️⭐️ 3/5 stars. Gorgon terminators from the Iron Hands who went a bit too far? Sure, that can work.
The Legacy of Nikaea: ⭐️⭐️⭐️⭐️ 4/5 stars. This is what happens when you have your Thousand Sons (or others) going around the outer reaches for too long and discovering what happened back home whilst they were away. They've gone a bit mad.
The Broken Helix: ⭐️⭐️⭐️ 3/5 stars. I freely admit to being lost with this narrative. They are veterans, but also either clones or aberrant? Are they maybe early Emperor's Children clones? There are some combinations that work here, but the narrative puzzles me. I'm sure others will have different takes.
In Disgrace All Are Equal: ⭐️⭐️⭐️. 3/5 stars. Yes, this one works nicely. Think of Dark Angels and I think the vibe here is obvious. The rules mesh nicely enough as well.
The Taint of the Xenos: ⭐️. 1/5 stars. Those of you who care about narrative like I do will find this combination hard to stomach.
The Weapons of Desperation: ⭐️⭐️. 2/5 stars. I want to like this more, but it strikes me that veterans should also have great arms. Perhaps they were in the cells of enemy starships and escaped? Who knows. Its an average combination at best for me.