(Not So) Legendary Actions is a certified DM's Guild Masterpiece by the Going Last Tabletop Gaming Podcast!
This is an incredible and absolutely necessary ruleset that has been posted by author Igor Moreno for free, and... are you still reading this? Go get this NOW.
We all want solo enemies to hold a threat over an entire party. It's a common enough media trope that we of course want to see that villanous last stand that causes the heroes to strive to the heroic maximum to succeed. Unfortunately, D&D isn't balanced with that specific moment in mind. A single enemy is either completely overwhelming with spells that the players can't handle or a complete pushover once they tear through its hit points.
Moreno took a cue from the Monster Manual's lich, which has Legendary Actions. These allow an enemy to act outside of their own turn, with limits on what they can do and how often then can do it. That turns a solo enemy into a reasonable fight! By scaling these ideas down, Moreno has created a means for even the weakest solo creature to become a meaningful threat.
You need this. This ruleset is smart and simple and every DM should use these to make their villains more of a threat during combat. And it's free? Free. If you haven't picked it up by now, I really don't know what else to say. It's that good.
You can hear more of my thoughts about (Not So) Legendary Actions on Episode 228 of Going Last!
[5 of 5 Stars!]