This is a scheme, planned by the Ulquaan Ibasa, a nation populated by undead who fled the Necromancer Wars, aided by the Cult of the Dragon Below. They feel Eldemar is weak from the Splinter Wars and the Weeping and seek to do much damage.
Teleport wards were internally subverted, artillery mages with deadly defenses bombard the city, fire elementals were summoned, foreign mages teleport in and attack various enemies, trolls are brought in from underground to attack civilians, winter wolves, giant brown worms, iron beaks as well cranium rats spy on city to find dissidents. The Cult of the Dragon Below supplies mages to attempt to summon a Primordial and kill people.
Originally demon summoning was also planned, but the time loop, which involved separation of the spiritual demonic realms from the human ones, stopped that.
The attempt to summon the primordial was rather poorly conceived of. While the primordial may have been landborn, primordials aren't summoned, they are awoken as the priesthood knew. The primordial wouldn't run out of mana and would keep rampaging until slain, something that would have been very hard to do.