Home-Field Advantage

From the introduction:

Lair actions, introduced in D&D 5th Edition, give shape to epic battles by introducing new gameplay mechanics which the players will associate with a boss monster for years to come. However, despite the fact that player characters face off against boss monsters at every tier of play, only a small portion of creatures, most of which in the 3rd and 4th tiers of play, were given lair actions. This book provides a collection of over 250 unique sets of lair actions, covering more than 320 of the most iconic D&D creatures, so that you can create thrilling boss battles for every tier of play, and breathe new life into some of your old favorites.