Legendary Resistances, Legendary Actions, Lairs, etc Building bosses https://dmdave.com/boss-monster-fifth-edition/

Boss Encounters / Boss Creation https://www.youtube.com/watch?v=uufG7zVE3nA https://www.youtube.com/watch?v=qoWr6PMPlhg https://www.youtube.com/watch?v=k2M4Zf6TSgs

https://halflinghobbies.com/the-dms-guide-to-using-legendary-actions-resistances-5e/ https://www.cbr.com/dnd-5e-legendary-lair-actions/ https://tabletopbuilds.com/legendary-resistance/?utm_source=rss&utm_medium=rss&utm_campaign=legendary-resistance https://tabletopbuilds.com/legendary-resistance/?utm_source=rss&utm_medium=rss&utm_campaign=legendary-resistance

https://slyflourish.com/building_legendary_creatures.html

Don't make the same mistake I did the first few monsters I ran with legendary actions. All three legendary actions refresh at the start of the monster's turn. This means you can use up to three legendary actions between the monster's turn (not one per round). Occasionally, there are legendary actions that may take up 2 or 3 actions but they are more powerful than the single-action actions.

https://www.reddit.com/r/dndnext/comments/2m22m6/list_of_legendary_creatures_by_cr/

Damage per turn calculations by DMG rules assume that

All attacks hit
All saving throws against the monster’s powers fail
AoE effects damage 2 creatures.