DREADMARES SPAWN ON EXT IN THE WASTELANDS STARTING AT 20:00.

I had most success finding pairs in the sulfur pits near the boarder/sunken Forrest, outside the snow/desert biomes.

YOU CANNOT PLACE ALTERS IN THE CITY OR BIOMES.

For a safe boss battle with the dreadmare/stallions, I use that giant flat pointy rock outside the sunken Forrest. Nothing spawns there.

It does fly around so just stay in place and wait for it to come to you. Once you fight it multiple times you have to click the alter for it to spawn.

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