Did you know that in 3.5 D&D there is a shark that lives and swims in acid. Isn't that awesome? It's a templated creature so you can even make other aquatic creatures that live and swim in acid. You know, like an elasmosaur. Nothing better than an acid pit full of sharks than an acid pit full of dinosaurs. Now lava is a little different but there's still D&D monster from elemental planes of fire or flamey death that live happily in lava. So stock up your moat full of lava and lava monsters. The heroes will love it or burn trying to cross it.
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