I don't know about that. Leknaat did nothing to stop him the entire game, not even her usual "advise the 108 Stars thing, so why would she start then? There's a whole other thread out there about why she didn't do anything, but my personal view is that she knew she wasn't strong enough to.
Luc himself says that his ritual will destroy the Grasslands and half of Harmonia. I don't remember if he mentions Zexen, but I would assume that they'd be nuked, too. Dunan might be affected slightly in the Matilda and Greenhill regions. Toran, Island Nations, and Falena would be unaffected (by the initial blast, at least). Depending on what actually happens during the ritual, there may be "nuclear winter," with dust and debris in the upper atmosphere. That we don't know. We also don't know how the balance of magic itself would be affected. If the True Wind Rune was destroyed, would that eliminate Wind elemental magic? The Suikoverse is an extremely magical universe. Would that cause damage to the fabric of their world? Luc says the world is only 1,000 years old. You don't reach that level of development after only 1,000 years without magic being heavily involved.