Legendary Bestiary: Legendary Actions for Low-Level Monsters

From the introduction:

Legendary actions are one of the coolest, most memorable aspects of fifth edition for me. As a DM, they allow you to make dynamic choices throughout a combat round, not just relying on a single action to make a huge impact. This element of unpredictability keeps the more tactics-minded players guessing as well, which is likely a big reason they come to the table.