Miasma Infestation
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It definitely wasn't something that came with the Dungeon, it had been formed here while I was away. This blob-like black slimy thing, also had a very nasty aura, and it seemed to burn through the grass around it, which had withered. It was definitely Miasma.
Miasma is a form of corrupted Mana and Lifeforce, decaying and dark, it often has cursed properties that can sap the life and mana of anything it touches. But it is also immensely powerful, producing a lot of energy.
It is often what fuels Dungeons, and also Demons can wield the power of Miasma, as their Continent is full of that, apparently. I am not saying its evil or something, but its still a dangerous and poisonous substance.
"Alice! Why the heck is there miasma in my Dungeon?!"
"I-I don't know! We'll need to first investigate what it exactly is… Ah!"
She suddenly pointed at the distance.
In fact, all around us.