Multicaster Rituals

[Eltab]

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.

 

[Alexemplar]

They're Skaven, right?

They have a machine- magical or otherwise- that can do this. Yes this is a big deal because this is the first time they've ever sone this. How? What else could they do? How can the party stop them?