While most combat encounters will be the heroes against a group of monsters, occasionally a solo fight is demanded by the story. However, when facing only a single enemy, heroes can pump out serious amounts of damage and take down even an extremely powerful foe quickly. So, solo monsters should almost always be Legendary.
Heroes should know when they are fighting a Legendary creature. It should not happen every session; these encounters should be saved for a particularly meaningful fight. This is THE SCARY DRAGON, or the named boss, or the Big Bad Evil Guy. A Legendary monster has a few important elements:
Legendary monsters act after EACH hero’s turn. This creates new tactical options and challenges for the party and aids you in balancing an encounter for different party sizes.
They have interesting mechanics and weaknesses that can be uncovered through research ahead of time or using the Assess action in combat.