A legendary creature might have a section describing its lair and the special effects it can create while there, either by act of will or simply by being present. Such a section applies only to a legendary creature that spends a great deal of time in its lair.

Lair Actions

If a legendary creature has lair actions, it can use them to harness the ambient magic in its lair. On initiative count 20 (losing all initiative ties), it can use one of its lair action options. It can't do so while incapacitated or otherwise unable to take actions. If surprised, it can't use one until after its first turn in the combat.

Regional Effects

The mere presence of a legendary creature can have strange and wondrous effects on its environment, as noted in this section. Regional effects end abruptly or dissipate over time when the legendary creature dies.