Daunting at first glance, this boss fight is actually pretty easy -- that is if you go in prepared.
First off, before you even step through the fog gate, what you need to do is go the Central Earthen Peak bonfire. Use the bonfire to light a torch. Now across from the bonfire, you will see a large windmill spinning. Go as close as you can to it with the torch equipped, and you will see an option that says "set fire". Do this, and most of the poison in the area will drain. This helps immensely for this battle, as the room is filled with poison. Keep in mind there will still be -some- poison, but only on the outer rim of the staging area.
The battle itself is somewhat manageable. It's better to equip light armor so that you can dodge attacks quickly. Usually dodging to the left or right should suffice in evading most of her attacks.
Killing her nets you 'Mytha, the Baneful Queen Soul' and 20,000 souls
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Mytha, the Baneful Queen is a boss in Dark Souls II. Located at the end of Earthen Peak.