How does PermaWar sound? The elimination of a node doesn't seem like it would piss off people on the other side of the world- after all, they are just portals to realms of infinite mana. I have a hard time seeing the elves get pissed that you destroyed the Clan's Death Mana crystal...
I understand some people wanting it to be temporary, however at that point it essentially is the same as Dispel Magic. The idea is that the Raven Guard are trying to permanently cut off access to the planes of force created by Agrares.
The Amurites will have a permanent negative relations with the Raven Guard due to their ideology.
In terms of power, I assume you are saying in the hands of other players/AI? Its quite simple - never have open borders with them if they are capable of destroying the nodes. Since the unit that can do this in enemy lands is only available at the same point as Archmages, its a fairly late-game power. Being able to use it offensively, the Raven Guard are a threat once they invade, but their unit limitations should prevent "stacks of doom" from descending on your mana nodes. The intent is to allow the RG to face against enemies which are heavily reliant on mana, as they have NO access to any spells or religious powers. A variation on scorched earth, essentially.
However, if there is widespread hatred of this, I suppose a Improvement variation could work.
Blocked Node
25/50/75/100 turn duration
Replaces the existing Node
Cannot be dispelled
Reverts to Raw mana at the end and can be improved.
Thoughts?