I do think it's a good idea to make the 1406 limit a spell, but I absolutely agree it should be undispelable, or even something like plane shift, where once it's cast once it unlocks shadow demons planar movement skill. It could be a special spell type, so that retorts which make global enchantments, or any spell of any sort more likely to appear sooner or cost less don't accidentally interact with it in anyway. No accidentally divine ordering for it.
Another good thing about making it a spell, is that you could do something like make it the first rare spell (Or 2nd or 3rd), then only let rare spells show up after year 1406, then scale the year by the mapsize, or add a new setting that scales research costs which could scale the year in which rare spells unlock. etc etc. I think you want it to be much more flexible than it is in CoM 1, but that flexibility should exist only for the game designer and game settings setup. Not for the wizards strategies or gameplay. Aka, a good way to make sure 1406 accurately reflects what's fair for various mapsizes and opponent counts and such.
Another good thing about making it a spell, is that you could do something like make it the first rare spell (Or 2nd or 3rd), then only let rare spells show up after year 1406, then scale the year by the mapsize, or add a new setting that scales research costs which could scale the year in which rare spells unlock. etc etc. I think you want it to be much more flexible than it is in CoM 1, but that flexibility should exist only for the game designer and game settings setup. Not for the wizards strategies or gameplay. Aka, a good way to make sure 1406 accurately reflects what's fair for various mapsizes and opponent counts and such.