Eltab
Lord of the Hidden Layer
Forgotten Realms 3e had 'ring magic' where several casters focused power and spell slots on one individual so he could cast a very powerful spell. I don't remember what sourcebooks to find this in, but the Forgotten Realms Campaign Guide and Unapproachable East might be places to start.
As noted above, you don't have to write rules for a Plot Device, you can just describe it happening.
As noted above, you don't have to write rules for a Plot Device, you can just describe it happening.