[Skarg]

While clever and not unreasonable, and I'm sure people could enjoy handling them this way, I think GMs aren't required to look for such an explanation to give ghosts abilities (or to have whatever artifacts they want), nor do I think these are intended or the way I'd naturally interpret what's written.

So, if I may offer other another views for contrast without coming off as bursting your bubble:

I've rarely wanted there to be weapons which were intelligent, or which had their own agendas, or could cast spells. Rarer even than I've been interested in having ghosts in TFT games, which has been extremely rare.

The vanishingly few times I have been intrigued by a description of an intelligent sword, it was in a savagely overpowered adventure for another game system, and my interest was about what it would be like to mix the down-to-earth rationality and typical power levels of TFT with crazy powerful-but-deadly-and-subversive magic as found in such adventures.

In those cases, however, it seems like the intelligent swords hadn't been staves that were now haunted by their dead wizards. The games with such adventures seem usually not even to have any explanation for how magic items (or monsters, or traps, or much anything) exist, or the explanation is supernatural, the mystical hyper-magical underworld materializes adventure stuff itself, or something similar. In TFT, I'd think they'd be enchanted by some very powerful wizards. But your idea is interesting.

I think the RAW ghost description default is that plain ghosts just can't cast spells. The generic ghost description says it can't harm except by scaring.

If a TFT ghost wizard did haunt its staff as a natural consequence of the TFT rules, and have its TFT-wizard spells available it seems to me that'd tend to be rather unlike any of the intelligent swords I've seen from other sources. It'd be rather like there being a wizard (with typical wizard spells...) that was rather hard to identify or kill. As you point out, mana would be quite a limited resource for it.

So that seems more to me like a ghost which can cast spells (I'd call that a variant of the usual TFT ghosts) and which can use its staff.

To get an "intelligent sword", I'd think it'd be about conjuring or imprisoning some other spirit into an enchanted item using researched spells and enchantments, and perhaps Geas on the spirit to get it to behave as intended (though then someone might think to dispel the Geas on someone else's sword, freeing the spirit to behave differently). But I'd want definitions of how it's possible for an object to communicate, and what such a spirit can actually do. Maybe a way for the holder to give their ST to the spirit.

If a spirit does Drain ST, what's its capacity to do so, if it has no ST itself? RAW, seems like zero to me, since the spell description just says it restores fatigue.