Gravestone/1.03.24

Dark Elf
Gravestones produce only Skeletons, which by being produced from their own building can free up the building queues for your Tombs, assuming that you plan on using Skeletons. Unlike Undead, the Dark Elves can't get much use out of Skeletons and must rely on their missile fodder role to have any need of them. However, as Skeletons are produced rather quickly making only a couple of buildings necessarily to mass them, unless you plan on producing Spiders / Queen Spiders along with them, then you wont need to build Gravestones at all.

Undead
Gravestone may only produce Skeletons, but as they can become any Undead infantry, that is all the Gravestone needs to produce. With its very small size and cheap cost that doesn't use gold, the Undead player can build many of these quite easily and within a confined space. Unfortunately, Gravestones only offer a single point towards the population cap, meaning you'll need to build MANY to support your army size. Interestingly, the Gravestone itself can be morphed into a Skeleton. At first this may not seem like a big deal or anything useful, but if you happen to be unfortunate enough to have your gold mines or / and your ore mines go dry (the resources needed to build Skeletons), then you can still produce Skeletons through essentially "swapping" the resources used to build them by building Gravestones instead and then morphing those into Skeletons.