Um, perhaps I am missing something, or perhaps its been too long since I was a PC rather than a DM, but . . .
Don't clerics get their spells (lists) from their deity? Wouldn't a cleric only item that granted additional spells / domains in effect be allowing a cleric to draw spells from another deity? Wouldn't their own deity perhaps have an issue with that?
I mean, I could perhaps understand if the cleric, say, wanted to have access to another domain of their deity (or even just the domain's spells). This could in fact be a notable quest for a lost holy item of their deity. Either the item itself will grant access to the domain's spells or the reward for finding and returning it to the primary temple of their deity is adding those spells to the cleric's list (not all that broken, considering that new cleric spells are being added to all clerics' lists every month or two, presuming the DM allows spells from Spell Compendium and whatever book he or the player has recently bought - which I admit is a very very big IF).
The problem I see is if the domain the cleric wants access to happens to be held by a deity other than their own. How do they justify this - in game? Why wouldn't they diety see this as the cleric seeking / attempting to convert to another deity? In fact, by seeking to find a way to use those spells, the cleric is in effect saying their patron deity is not strong enough (in the magic department) for them - a rather heretical thing to say, I would think.
The only way I could justify allowing a player to do this - in game - is if the deities are allied or otherwise close, and the cleric is seeking to be a priest of both, in effect akin to an intermediary for the two religions. In such an instance the cleric would pray to both, and would lose one domain to take on the domain of the other deity. In Faerun I would perhaps call to mind the Triad as a potential example, as I think clerics can worship the Triad of warrior type deities as a whole and choose domains from the collection rather than a singular deity of the Triad (Tyr, etc). It's not quite what I have in mind, but it is close.
In any other situation - such as seeking domain spells from a deity enimical with one's own deity - I would have the cleric receiving warning nightmares from their deity about their straying from the correct path, etc. Agents of the other deity might seek out this potential convert, promising power (ie: access to those spells, and more [the domain power, etc]) in exchange for turning to the new deity, etc. Eventually, if the cleric continued to seek access to those spells, they would lose their spell casting as their deity turned away from them. They would then need to either convert to another deity (likely the one they sought the spells from) or seek atonement from their own deity. Either way, they will not be mixing and matching domains of different / non-allied deities.
Or am I just being short-sighted here?
