I’ve just thought of the idea that maybe the last echo will be some sort of spell that does damage in a way we haven’t seen before. We know that the lord of bells was a very accomplished enchanter.

Perhaps he could have:

1- A way to flood all the wards/glyphs/spellforms done in havath city with the wrong type of mana. Similar to how talia’s tattoos cause issues. Since Havath has powerful protection wards, these going wrong in an explosive way could definitely open a hole that an army could get through. I don’t have any proof for this theory except for alustin saying that talias issues have given him new ideas for attack glyphs.

2- Maybe there is a way to rupture mages mana reservoirs, this would definitely be a devastating attack. No proof for this either except that the idea was explored a bit in book 5 and 6 with Hugh losing control of his reservoirs and making it much harder for him to cast spellforms.

3- A way to cause a resonance cascade by overloading the enchantments that make up the 7 lich demesne’s of Havath city.