So, the text of find familiar reads:
You gain the service of a familiar, a spirit that takes an animal form you choose.
The key word here is "spirit". One of my players, for example, is a warlock, and semi-for-flavor has a fragment of a forgotten god carried along with her. It's a long story, but the point is, she thinks the "spirit" that her familiar is, is a piece of the "forgotten god".
She subsequently thinks about the text on the spell that says:
As an action, you can temporarily dismiss your familiar. It disappears into a pocket dimension where it awaits your summons. Alternatively, you can dismiss it forever.
She wonders if she can, by banishing the familiar, send the familiar and whatever it's holding (e.g. a powerful magic object) back to the place it came from – in this case, the forgotten god, for him to feed off of.
My question is: is there an exact definition or ruling for this "spirit"? I thought it was like a will-o-wisp, or like a lost animal's soul, or something. Could she actually make this spirit a fragment of a larger being's spirit? Or is that more up to the DM's judgement? Honestly, I'm the DM, and I'm confounded by this.
Best Answer
The spirit is up to you, the mechanics are generally determined by the spell.
As a DM, you can absolutely work with your player to utilize a spirit that fits with their design. You can always go much further than that, but in general, the mechanics of find familiar are clear.
Pocket Dimensions are separate
This question covers more about pocket dimensions and what can/can't go into them and what that pocket dimension entails. You always have the right as DM to expand on that, but if you feel that by doing so it creates problems with your world or gives a character more capabilities than they should have, then you can stick with the rules as written and keep it limited.
The takeaway: I included this to specifically address the question on bringing something back to the pocket dimension. You can't unless you as the DM allow it. Find Familiar functionally would just call this fragment of the god, which resides in it's own pocket dimension, and send it and it alone back when dismissed or killed.