Warpstone Flux Rating: ⭐️⭐️⭐️3/5 stars. The rules are reasonable, but not as outstanding as they might otherwise could be. The combinations with other oaths are like marmite: either great, or terrible.
Background.
The Imperium said not to touch. The Mechanicum said to just destroy such things. But you, a Blackshield, are no longer bound by such pettiness. You will happily scoop up that Eldar blaster thing, or other technology and use it to fight with. They are effective, after all. But at what price?
Rules Evaluation.
I really like the narrative of this rule and the conversion opportunities. Eldar (Aeldari, and Drukhari alike), Necrons, and perhaps even Orks and - in extremis - Tyranids are all viable conversion opportunities for a force that is wielding alien technology like its not a big deal.
You can exchange weapons if you would ordinarily have plasma guns (which honestly might be better to just keep unless you get in 18 inch range); nemesis bolt guns (which might be better at range); a combi weapon (which is a viable and sound swap); a plasma pistol (which is also a viable swap); or a power weapon (which is a great swap for the S bonus) for xenos weapons.
The Deathlock weapon has a solid 18inch range with assault 2. Although the plasma gun is strictly better in terms of S, the rate of fire from the deathlock is superior, and deflagrate makes up for the S loss somewhat. The loss of AP compared to the plasma is a bigger issue though and this is what makes swapping plasma guns for deathlocks a bad move. Nemesis bolt guns are a viable swap if you don't care about the range. The combi bolt guns are another story, and I'd strongly take this option.
The Doomlock weapon is half the range, and is basically the pistol version of the Deathlock. As an exchange for a plasma pistol, it works in terms of the rate of fire, but again suffers from the AP loss which is a concern. A bit of math hammer here is needed, but I think the result is that the plasma pistol is better.
The Halo Blade for the power weapon is a great choice due to the S+3 and AP=3 characteristics in comparison to the usual power sword. Not so great compared to the power axe in terms of AP, but the S gain is impressive enough to do this wholesale.
The downside of the ranged weapons is the Lethal Exposure rule. You will lose models to this until your unit is small. Hence this rule calls for multiple small units where possible (but this invokes the hidden "tax" on buying such small units - making small units bigger is cheaper than multiple small units in Heresy games).
So, what to do then. I think Veterans are where this option really shines brightly, as well as terminator squads, and perhaps assault squads. I'd be keeping plasma guns otherwise.
Combinatoric Analysis.
The Eternal Vendetta: ⭐️. 1/5 stars. Doesn't work for me, sorry.
Panoply of Old: ⭐️⭐️. 2/5 stars. Also doesn't really work for me. Two sources of alternate weapons might be interesting, but it is confusing too.
Only in Death Does Duty End: ⭐️⭐️. 2/5 stars. This one continues a line of secondary choices that doesn't fit the narrative for me. Sorry folks!
The Spoils of Victory: ⭐️⭐️⭐️⭐️. 4/5 stars. Finally we come to the first pairing that really works! Pirates using xenos weapons is very fitting. How about some Eldar weapons that you stole? It'll be just like the Rogue Trader description of what the Eldar were like!
An Eternity of War: ⭐️⭐️⭐️⭐️. 4/5 stars. Another good combination to my mind!
The Flesh Is Weak: ⭐️⭐️⭐️⭐️ 4/5 stars. Dornian. Heresy. Iron. Hands. Need I say more. This is all shades of awesome. Do it. I need to see this done. Please? Pretty please? You've got me begging.
The Legacy of Nikaea: ⭐️⭐️⭐️⭐️ 4/5 stars. Psykers using Xenos weapons fits extremely well. I can see all kinds of Thousand Sons or Blood Angels corrupted forces for this one.
The Broken Helix: ⭐️⭐️⭐️⭐️ 4/5 stars. Mutants and clones who picked up alien weaponry? Sure, why not! I could even imagine some kind of genestealer cult aberrant conversion here potentially.
In Disgrace All Are Equal: ⭐️⭐️⭐️⭐️. 4/5 stars. Disgraced already? Then there's absolutely nothing stopping you grabbing those xenos weapons is there? A great combination.
Pride Is Our Armour: ⭐️. 1/5 stars if you care about narrative. Why would veterans of the Great Crusade who still hold on to those ideals pick up alien weapons? However. ⭐️⭐️⭐️⭐️⭐️ 5/5 stars if you are just looking at those juicy rule combinations. Those veteran squads were made for the taint of the xenos. I'm sure there must be a narrative of why veterans are using such weapons. I know: they fell in with the Interex! Narrative sorted.
The Weapons of Desperation: 0/5 stars. No need for this combination - you already have xenos weapons and you MUST swap weapons according to this second oath. Take something else. This combination doesn't work.