Hi, so I'm making a character who's a wild magic sorcerer named Pandora who worships Mystra, the deity of magic. I want her to have a pet cat who is actually Mystra in disguise (the cat's name is Circe). I'm keeping the physical stats mostly the same as a regular cat's, except that I gave the cat 120 ft. of truesight and telepathy, as well as making the intelligence score 30, the wisdom score 25, and the charisma score 27. I also added these two things to a cat's stat block in the Monster Characteristics Description section: 

Mythic Creature. When Circe drops to 0 hit points, she disappears, leaving behind no physical form. She reappears after 2d12 hours. As an action, she can vanish from her current plane of existence and appear in the Ethereal Plane. As an action while she is in the Ethereal Plane, she can reappear in any unoccupied space within 30 feet of where she disappeared. Whenever Circe drops to 0 hit points, she leaves behind in her space anything she was wearing or carrying.

Goddess in Disguise. As Circe is actually Mystra, the deity of magic, she can cast any spell ever known at maximum level and have multiple concentration spells happening at the same time. The only exceptions to this are if she casts wish, time stop, or gate, which requires enough concentration that no other spell can be cast in combination. She also can shapechange at will and can grant other creatures spell knowledge by touching them. She controls and provides the Weave, allowing (relatively) safe access to the power of raw magic to mortal spellcasters and magical craftsmen.

(Some of the ability stuff I copypasted from the Mystra article on the D&D Wiki) I would publish the entire homebrewed stat block so people could understand it better, but once I do that it's published forever and I can't take it down. I know deities in 5e generally aren't supposed to have actual stat blocks, but this is different since it's a deity taking on a mortal form. (sort of)

Here's what I need help with:

1. what damage adjustments/condition immunities should I add

2. Is there anything else I should change

3. and does anyone have any other advice

You really don't need to.

Of course if this is a project you want to invest it, sure! Go for it. But as it's in the realm of the divine, you can just have the being do whatever it is you want it to do. You don't have to make a formal writeup.

I would make it a fully normal cat, or fey creature, except during the times it is possessed by the deity. During that time it can pretty much do anything the deity can do.

A player with constant direct access to a god is a dangerous thing in all gaming respects.
First off the character will. and they will, come to rely on the gods intervention in dangerous times.
Second off the DM will use it far more than they should for any number of reasons.

Yeah, this is a ridiculously OP ask from a player. Way too much power at hand. If as a DM you want to have deity dipping a toe in, that's really more just a matter of pure DM fiat. Could hash out a stat block if you really wanted to, but they basically can do whatever you want.

I have to agree that this sounds insane.  Obviously it’s not impossible it makes sense somehow within the Campaign, but players in your average game aren’t going to be statting out their deity and then making it a pet.  I can see someone within an assistant DM position statting out a god for fun and proposing it to appear somewhere in the DM’s world, but this?  Hell no.

No.

Compare your desire to how Divine Intervention actually works in the rules:

Divine Intervention

Beginning at 10th level, you can call on your deity to intervene on your behalf when your need is great.

Imploring your deity’s aid requires you to use your action. Describe the assistance you seek, and roll percentile dice. If you roll a number equal to or lower than your cleric level, your deity intervenes. The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate.

If your deity intervenes, you can’t use this feature again for 7 days. Otherwise, you can use it again after you finish a long rest.

At 20th level, your call for intervention succeeds automatically, no roll required.

You want a pet to endrun the parameters of an extraordinary and literally dicey feature belonging to other classes.

If I were your DM, like maybe if your character was starting at tenth level, and you forgo what would have been otherwise your character's feats or ASIs, I might grant the sorcerer character the ability to use divine intervention via asking the cat for intervention; but that would also require a very generous mood on my part.

When you're a player, simply put you don't get to just declare that your character has a pet that's secretly a god and has all sorts of nifty powers.  Trying to push the issue is likely to get you kicked out of the group.  A player character is already an extraordinary individual, trying to insist that they get something like this is when terms like "Mary Sue" start getting thrown around among gaming groups.

When you're a GM, you don't need to stat out a god in order to have it appear, but you really, REALLY shouldn't have a god just accompany the party and pop out super awesome miracle deus ex machinas.  That is incredibly annoying and feels like showing off. 

Oh! I thought the OP was the GM not a player.

If you want anything even a fraction like what you were talking about you have to talk to you GM and don't be surprised when they give you a solid no.

I would consider giving you a familiar tied to a magic item and compensate the rest of the party in some way. Then you can role play that your PC thinks that the familiar is the representation of a god.

To be fair, it's unclear; this could be a NPC/monster stat block, and I think some DMs may want to mechanically gamify a way a deity might function, and they do use "stat block", so unclear if that is for a NPC/monster or a sidekick.  I think the advice given in this thread for a divine stat block or lack there of is great, but if this is a player option, and we've all seen player's propose options like this, it's definitely a non starter or at least a deep collab that winds up more under DM control than player.
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