[RPG] What are Mythic Actions

No, even monsters only get one action per turn, and actions should not be confused with attacks. in the case of monsters that can attack multiple times, that's because Multiattack is one action, that just happens to result in attacking more than once. Each attack is not an attack action! Multiattack is also very specific about which attacks the action results in.

So an owlbear can indeed grapple or shove, but that grapple or shove is done by taking a normal Attack action, not a Multiattack action. The Attack action is general purpose and more flexible than Multiattack, but doesn't allow the owlbear to make more than one attack during the action.

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.