But, if the God is an Undead, and LG, then how can that god declare undead to be evil? Why would he provide a form of the spell to create undead that is evil? Why would mummies made (I believe Create Undead does make mummies, but I could be wrong) in this God's purview, be evil monsters bound by dark magics of evil gods?
1) I don't share your interpretation of Osiris that he was worshipped as being an undead god. He was killed, resurrected, and rules the afterworld but I don't think that his priests or worshippers identify him as undead.
2) Osiris did not declare undead to be evil. Skeletons and zombies and mummies are evil under the D&D ruleset because of their behaviour. They are willing and happy to slaughter innocents and suchlike - which means an evil alignment in D&D.
3) I've gone into why a Lawful Good deity might allow the creation of a Lawful Evil being to serve a specific purpose. If mummies were good-aligned, they would make less-capable instruments of divine punishment.
(I also believe that D&D mummies are based around the folklore of tomb guardians and punishment of desecration rather than the actual Egyptian mythos.)
The point isn't "PCs can copy what was done" but that if that is possible, then why are lesser forms of good undead not possible?
For the same reason that you can't cast Fireball as a Transmutation spell, or make it deal Lightning damage by default. That is simply the way the default D&D setting and mechanics work.
Skeletons have very rarely been able to talk across any media. I'd say you have a better debate with zombies.
We're talking 5e D&D. The Animate dead spell doesn't say that Skeletons can't talk: that is only spelled out in the MM. Just like the Animate dead spell doesn't say how an uncontrolled Skeleton behaves, but that is spelled out in the MM. The parallel is there and a player might use a similar argument that the absence of that detail from the spell text or statblock in the PHB justifies that when they raise a skeleton using animate dead, it can talk.
But that is part of the problem here isn't it? There is only a single sentence that players have access to calling Animate Dead evil. And nothing about the spell itself gives a solid reason for it to be evil. You need to have read the Monster Manual, and accept that the Monster Manual is giving an accurate picture of all instances of skeletons and zombies. Which it isn't. After all, players can't make Ogre Zombies or Beholder Zombies, even though they are covered by the same lore and very similar mechanics. Same with Warhorse Skeletons or Minotaur Skeletons.
"Foul mimicry of life" is reasonably descriptive of evil.
I think that trying to claim that the MM is not accurate in this case because the Animate Dead spell does not allow the creation of Skeleton variants is a mistaken argument at best. - It specifically states that Skeletons can arise by both spell and spontaneously.
Do you have access to the MM?
So, I feel it is fair to question, if the MM is not 100% accurate as to what is going on, and the player has no information from the MM anyways, why must we assume that the character is committing an evil act by binding a hateful spirit to the dead body? The player has no way to know that unless the DM tells them, and I can't think of a very good reason the DM would force them to do so, if they could instead bind a good spirit of a protector to the body.
There is no reason to believe that the MM is not accurate in this case.
Even if the player misses the text in the PHB talking about creating undead the Animate Dead spell itself talks about "foul mimicry of life" and the statblocks in the PHB state that Skeletons and Zombies are Evil.
This is not "no way of knowing" and I do not think trying to make that claim is correct.
The DM is as always at liberty to tweak things, such as allowing a good spirit or the original soul to animate a skeleton, if they so choose.
They are being used synonymously
The Raven Queen is a "dark" goddess, but not called out as an Evil one for example.
But why must they be evil? Osiris is Lawful Good, the canopic jars referenced by the Mummy Lord have always depicted Good deities from the pantheon, the sons of Horus.
I'm not saying you can't make an evil mummy, I'm asking why you can't make a good one?
A PC can't by default because they are using a spell designed to create a relentless avenger of profane transgressions. If a PC wanted to research a spell with a different purpose, they should talk to their DM.
But why are they evil if there is no reason they must be evil? What prevents you from using necromancy to bind a willing, good soul to a body and make a good zombie?
The default D&D mechanics and setting. - It is just the method that spell uses the way the world works to achieve its purpose.
Ravinica has the Golgari Spore Druids, who make Zombies (using the zombie statblock specifically) from spores and fungus control. No evil spirits anywhere involved in that.
No, but the spore zombie will still be evil. - Probably because it no longer has any human impulses or ethics and will happily create more dead bodies for fungus to grow on.
And with the Deathless in Eberron, whether or not Zombies are traditionally evil or not, we know that it is possible to make Good Undead.
I don't think that anyone is claiming that it is not possible to make a good creature that is classified as Undead. IIRC this discussion is based off whether the Animate Dead spell might be viewed as evil, illegal, or immoral bear in mind.
The description of Deathless (Undying) makes it very clear that the origin and existence of Undying is very different to that of normal Undead.
