Death magic...
If your nation lacks magical lowlevel diversity, summon spectres [they had D?2]. (And start out by using them to sitesearch all your land with Dark Knowledge). These also make decent upkeep-free researchers. And with a few pathboosters, they'll be able to make other pathboosters [and remote sitesearch to 9 by spells], and before you know of it you've got a spectre summoning a troll king or sea troll king, and you've got access to 4-5 E or W spells, and....
...Admittedly, that particular aspect is less important for magically diverse nations like Arcoscephale, but spectres are a real strength of death magic for "focused-magic" nations - depending on game length.
If your nation is short on cheap researchers or you just want better research, forge skull mentors. [Using a dwarven hammer, of course]
If you'd like a strong thug chassis, summon a Bane Lord. And while he does come with the powerful Bane Blade, he really deserves a better weapon... Like a Wraith Sword... Which also takes death gems.
If you desperately need something to trample the legions of cheesy infantry your opponent is fielding but cannot recruit any trampling units? Death magic provides the answer in the Behemoth.
If you want some very, very, very nasty archers against an opponent that uses tough troops, the Banefire Archers are your friends. Of course, they are fragile and high-cost, so don't field them without many lowerlevel undead to absorb the banishments.
If you need an instant cavalry force that's not particularly tough but does good damage on the charge, just the thing to break an opponent's lines, death magic will provide it in batches of 20+.
If you want to bootstrap to higher magic in a long game - death is the way to go, only nature is somewhat comparable in strength. The skull staff, helmet, ring of sorcery, and ring of wizardry all boost death and the higher death magic you get on the way, the higher level death mages you can summon, which means that they, using the items used to boost death enough to summon them, can summon even better leaders, until you reach the spot in the late game where you've upgraded from D2 pretender through (possibly skipping some steps) spectres, mound fiends, wraith lords, demiliches, and into hardcore multiple tartarian summons per round [or massive ghost rider devastation or legion of wight spamming] while trying to keep your Well of Misery up in its place in the Global Enchantments. In principle, you just need a pretender with D2 to get this started - once he has summoned his first spectre, if you focus on bootstrapping death magic it will happen.
Additionally, though that's for the very, very, long game... There's something inherently cool in being able to summon powerful immortal commanders like wraith lords and demiliches, units you can cheerfully send to their death wearing minimal equipment [a few death gems, nastily scripted orders, and flying boots
] round after round so long as the fight is in your own dominion. Probably not cost-efficient in most cases, but cool.
I've often sat around with gems of other types unused because "there's nothing really useful to use them for at this level of research". That reasoning seldom, if ever, occurs with death gems as soon as you have your first D2 unit. Sure, you may save them, but only by a conscious decision to forego something truly useful in the now for something potentially more useful in the future.