(Note: my campaign is a low-magic setting that I run with Grim Tales, so I dropped a lot of the D&D assumptions)
In my homebrew, death isn't an entity. In fact, the gods aren't even "real" in the usual D&D sense. Various religions may look at it differently, but at the same level of truth as the one at which wizards do use magic to make things explode, and dragons breath fire, there are no* physical manifestations of gods--nor one for death--with adgendas, who communicate with followers.
So, religion is based on faith, not gaining the ability to cast spells. To be fair, there is a certain level of supernatural stuff going on involving the gods (mostly with fate/luck), but nobody gets spells. On the same vein, there is a certain luck/karma/fate/destiny sort of thing going on with death, in that there's a bit more poetic justice than on modern-day earth, but there isn't any sort of sentience behind it.
Now, at the same 'level of truth' I mentioned earlier, the afterlife is fairly simple. When people die, their spirit leaves their body, and lingers for a while, depending on how/why they died. Sometimes, through extraordinary means (very rare magic/rituals, or extreme willpower, or a reason to continue living), someone can return to their body, so long as they weren't dead (or mostly dead, depending on how they come back) for too long. If the spirit feels ready, it rejoins Yril (the life/nature goddess).
Undead come in two varieties; lingering spirits and animated corpses. The lingering spirits, obviously, are the sentient undead, who tend to be lingering on their own accord, for whatever reason. The animated corpses come to be in a much more evil way. You see, the latent 'spirit energy' that exists in the world can be manipulated with magic, and can be forced into the bodies of the dead. This spirit energy effects the corpses in a much more raw, powerful way than the natual spirits did in life, so skeletons can move on their own accord, but they're inherently violent and unthinking. The process works best around recent death (since the spirit energy is 'closer'), so necromancers do some pretty...bad things to make sure that they can get the best/most minions possible.
So...undead make the more ardent worshippers of Yril very angry, and they go out of their way to destroy them and their creators. This happened a lot more in 'ancient times' however, due to the near mutual destruction of both groups in more recent times. A sort of druid-necromancer war, if you will.
Well, that's all I have to say about [Death] tonight...I may think of more in the mean time.
* The 'earth goddes' is the planet (known as Yril), so she does physically exist--and she has an agenda: the cycle of life.