Ondra's Mortar is found at the very northeast edge of the archipelago and can be entered through Ofecchia Channel. It is protected by massive storms from anyone and anything that would want to enter. The rocky, volcanic islands of Magran's Teeth border the storms, holding them back from the rest of the Deadfire.
It is a massive expanse of uncharted sea, and between the relentless hurricanes, mile-high waves, and winds that can strip the hull off any boat, it is one of the most hostile length of ocean known to Eora. As winds and currents stir themselves up into naturally occurring storms, the radiating energy of Ondra's Mortar feeds them, evolving otherwise insignificant wind systems into fierce and unrelenting hurricanes.
The people of Rauatai know with certainty that Ondra's Mortar is responsible for some of their country's worst natural catastrophes, though such storms must cross a continent to assault their distant coasts.
Since the Deadfire's arguably most prominent, unique, and mysterious and characteristic is luminous adra, Rauataian scholars believe that adra was somehow responsible for the chaotic conditions. It is widely speculated that somewhere in the midst of those storms lies a wealth of luminous adra greater than any found in the Deadfire. However, the inaccessibility of the region doesn't allow anyone to confirm these suspicions. 
Points of interest
The lost city of Ukaizo can be found there.
- Pillars of Eternity II: Deadfire Guidebook