[RPG] a phoenix in D&D 5e

The limits of the action economy (1 movement, 1 action, 1 bonus action and 1 reaction per creature per round) mean that a single creature is always at a disadvantage in terms of the number of things that it can do with respect to a group of creatures (like a PC party). The mechanical purpose of legendary actions (lair or otherwise) is allow the creature to break these constraints. In effect, legendary actions are simply things that a creature can do out of sequence.

In terms of determining a creature's challenge rating, the fact that an action is a legendary one is irrelevant: it is just something that needs to be included with all its other actions/bonus actions when determining its damage output (for offensive CR) or effective AC (for defensive CR). For example, an Ancient White Dragon has a damage output of:

• Multiattack: Bite/claw/claw (2 rounds in 3) for 19 + 15 + 15 = 49
• Breath Weapon: (1 round in 3) for 72 x 2 targets (probably) = 144
• Legendary: Either 3 tail attacks for 17 x 3 = 51 OR 1 tail and 1 wing for 17 + 15 x 2 targets (probably) = 49
• Lair: Two options that each do 10 damage to everyone in a huge area for 10 * 4 targets (probably) = 40

This averages out to \$ (49 \times \frac23) + (144 \times \frac13) + 51 + 40 = 175 \$ for an offensive CR of 22. Yes, I know it has a CR20 in the book - this is just one of the many CR anomalies in the Monster Manual.

In general, legendary actions are not needed for monsters with a CR of below 10 or so because the PCs cannot generally fully utilize the action economy themselves when these CR are suitable solo opponents. The problem that legendary actions were created to resolve doesn't really kick in until the PCs are getting an action, a bonus action and a reaction almost every round. If you want you can add legendary actions to low CR monsters and boost their CR a bit but its not really worthwhile.

As an aside, Legendary Resistance despite its name is not a legendary action.

The fun way to do this, as well as the obvious interpretation of the rules, would be to have them imitate phrases perfectly, sounding like the original speaker, but using them in approximately the sense they originally heard them. So their voices can change completely from sentence to sentence - one being a dwarf they heard speaking yesterday, the next sentence being a small elf child they heard ten years ago, and the third being something you said to them three months ago.

They understand what phrases and sentences mean, but if they can't break down phrases into words and use the words individually, they're still fairly limited by their curse. They would sound a bit like Dilbert's pointy-haired boss, who tends to come out with phrases and clichés that belong in a subtly different conversation.

This would be an interesting challenge for a player who was usually verbally adroit. Precedents for even more extreme versions exist in the Librarian from Terry Pratchett's Discworld series who manages to get by with just one word, "Oook", and Mr Punch in the classic Punch & Judy show, most of whose lines are "That's the way to do it!"

The main problem with playing a Kenku this way would be avoiding humour in serious moments of the game.