Ok I think it's pretty much not possible. This just happens to be one of those things that FE/LH doesn't allow for. Obviously you can do this for a tactical spell, but not so for a strategic spell.
I don't actually think that UnlockUnitAbility works. It certainly doesn't appear to work for an item. I had a look at the Boots of Stealth which use this (they're supposed to grant the wearer the MasterScount ability) and I'm fairly sure that it actually doesn't work. You could probably log a bug report for that if you want but given it's about the only item that uses UnlockUnitAbility it's probably not about to be fixed in a hurry. Still doesn't hurt to try.
You can grant an ability via a spell, but that doesn't really help you.
You can have an item that is a one-use item grant you an ability, eg. like the Tomes grant Spell abilities see Tome of the Wind. But that's nto quite what you want. Even if you have a spell that grants an ability (or an item).... it is also very difficult/impossible to remove the ability.
You could make it so that your custom Corruption spell requires a special resource.... but the problem there is that to the best of my knowledge an item can not produce a resource..... an ability can, but then we're back to the same problem. So not really a solution there.
There is one thing that I think would work... but it would kinda suck. I'm fairly certain that you can set a minimum required UnitStat to be able to cast a spell. So what yo do is create a new UnitStat called UnitStat_StaffObession or whatever, then have the staff grant +1 to that UnitStat..... then you have your spell require the unitstat using <RequiredStat>UnitStat_StaffObession , 1</RequiredStat> in a SpellDef.
I think that would work, but even if it does it has 2 issues:
1) The spell would have to be Autounlocked for everyone, so you'd always see it in the spellbook... just greyed out until you equip the staff.
2) If you want this to apply correctly for your Sov, you have to change <IsSovereignExemptFromSpellStatReqs> to 0 (default is 1) in ElementalDefs.xml.... though you can probably do that with a file in your mod folder.
Depending on what you are trying to do there might be alternatives that would work easier/better. Where is the staff in question coming from.... do you plan on making it a random loot drop.... or something that is granted via a quest? Could you give out something like a Corruption Scroll instead... that when used grants the champion an ability that unlocks a Corruption spell? If you're giving the staff out via a quest you could just unlock the Corruption spell directly instead.
Hopefully something in that helps. But yeah, unfortunately sometimes with modding FE/LH even though an idea might not be possible so you need to go for a similar idea instead. There's a lot of things that you can't do with modding, but there certainly is a lot of things you can do also.
Let us know how it goes.
