Can a creature transformed by the True Polymorph spell use Legendary Resistance?

Question

Can a creature transformed by the true polymorph spell use Legendary Resistance?

The "Legendary Creatures" section of the introduction to the Monster Manual (p. 11) and the corresponding chapter of the basic rules includes this line (added in the first MM errata from 2015):

If a creature assumes the form of a legendary creature, such as through a spell, it doesn’t gain that form’s legendary actions, lair actions, or regional effects.

Which doesn't seem to state anything clearly and specifically either way regarding legendary resistances. So can a true polymorphed creature use Legendary Resistance?

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Link to relevant question about other legendary abilities: True Polymorph: can a player use legendary actions of its new form?

Answer 1

TL;DR Yes.

Supporting points:

Legendary Resistance is not part of the legendary actions. Legendary actions is a mechanic by which the legendary creature can take some "actions" while it is not their turn. Usually three legendary actions can be taken per combat round although some of these cost multiple actions.

This is an action economy resource that can be used for specific things. The legendary actions always appear as their own paragraph of the stat block detailing what they can be used for.

• Example: Vampire

  Legendary Actions The vampire can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The vampire regains spent legendary actions at the start of its turn. {Italics Mine}
  Move. The vampire moves up to its speed without provoking opportunity attacks.
  Unarmed Strike. The vampire makes one unarmed strike.
  Bite (Costs 2 Actions). The vampire makes one bite attack.

Legendary resistance is a trait that also appears on legendary creatures but is not part of the legendary actions paragraph.

• Example: Vampire

  Legendary Resistance (3/Day). If the vampire fails a saving throw, it can choose to succeed instead.

Legendary resistance and legendary actions are unrelated except insofar as they appear both on legendary creatures and are (causally related) similarly named.