Ah, death gems are the best! Here are some uses you might consider with Yomi:
-Horror helmets! The Dai Oni appreciate helmets, and the gems you have the most are death gems. (The added fear should rout your enemies even quicker, too.) Of course the 5 gem helmets from earth or nature would be more economical, but work with what you got.
-Black Bow of Botulf. These are way evil items, that can cripple most any thug/SC. Indeed, it would seem to me that none of the usual protections apply: mirror image, high-parry shield, high protection and air shield all have seemingly failed to prevent this from working in a very small test me and Ygorl did in a game. This could be the very best item in the game, in fact, for the research/gem cost and unstoppable power.
-Summon Spectre used to be great... I'm not sure what magics they can get in Dom3, but if they can get astral they would be very useful indeed, despite the cost.
-Manifestation is an excellent high-conjuration spell. It's pretty hard to cast, but well worth making a couple boosters for.
-Lichcraft gives you D4-mages. Give them a Skull Staff, and you have Ghost Riders -capacity for a reasonable price. As you can see, this is very late game indeed. Give 'em a couple more boosters and start making Tartarians while you're at it
Dai Oni with a death random can also be used for these purposes, so get Construction 8 and Conjuration 9 before Enchantment 8 for the demiliches.
Hmm, well, as you can see death gems are very much geared towards late game, at least in my strategies, with the important exceptions of the few items. Skull talismans are great on assassins, but Yomi doesn't have those. You can never have enough death gems once you can make liches, so just stockpiling them is a good idea. In fact, stockpiling them until you get Conjuration 8 and blowing them on a reinforced Well of Misery is not a bad idea at all. (You'll need a death-random Dai Oni, and a Skull Staff and a Skullface, so construction 6 first, though.)
The already-mentioned Ghost Generals are decent enough, but you're probably more limited by your items than thug-chassises; I should think you won't have your pretender and all your Dai Oni equipped very soon anyways, and they make for better thugs than the Ghost Generals would.
In Dom2 I loved Pale Riders, but sadly, in Dom3 it just doesn't cut it, I think. The same with Reanimate archers: these guys are rather useful archers, but they are just too expensive to make in the new economy. At least, in my opinion.