Multiclassing? Tieflings? Never Heard of 'Em! Official/Optional Rules to Consider "In (Your) World"

Did you know there are rules for madness in the Dungeon Master's Guide? Did you know that dragonborn (with or without tails) are an optional fantasy racial option? Let's also not forget multiclassing, feats, and so much more! When making up your TTRPG world, it's OK to in/disclude all these and more - but understand the impact it'll have at your table and in your world!

More thoughtful moments:
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