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Every Ancient Dragon has Detect as a Legendary Action:

Detect. The dragon makes a Wisdom (Perception) check.

Unless I'm missing something, this seems to be a worthlessly ambiguous action. PHB 174 (under "Ability Checks") reads:

The DM calls for an ability check whenever a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results.

In other words, after an action is declared, if it can fail, dice must be rolled. But the Dragon isn't attempting a fallable action; it's just rolling perception dice with no means or ends, with no attempted action attached to them. It's skipping the part where it attempts an action and it's going straight to the dice rolls.

Can the dragon attempt anything that the GM could justify as a perception check? Or does it have a more specific purpose, such as locating a hiding character or identifying an illusion? If so, why is it termed so ambiguously? If not, what does the Detect action actually do?

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It seems to me this legendary action to detect is an attempt to simplify the dragon's ability to make active perception checks (normally requiring your action on your turn) when it is not his turn using the standardized legendary action system introduced in 5e.

The difference between the standard wisdom (perception) check and the legendary Detect action is that the legendary action can be performed out of turn. This allows the dragon to, for example, detect a rogue who has hidden outside of the dragon's turn.

Normally, an active perception check is performed on your own turn during the combat round and uses your action for that turn. This would ordinarily grant a rogue a chance to hide, then perhaps make a sneak attack, before enemies have had a chance to actively search for the rogue.

Legendary actions, on the other hand, can be taken by legendary creatures after any turn. With this Legendary action, the dragon can detect the rogue after he has hidden, but before the dragon's next turn. It effectively gives the dragon a chance to find hidden characters out of his turn and without wasting his action on his turn to do so (assuming he hasn't found them passively).

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  • \$\begingroup\$ Does this suggest that Detect is not the result of the Dragon attempting to look for something, but rather just to mechanically interact with things that care about Perception checks? This is the first time I've ever seen a die roll that isn't prompted by a monster trying to do something. \$\endgroup\$ – Robert May 19 '16 at 17:26
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    \$\begingroup\$ The monster is trying to do something: detect. This is called out as a legendary action because it means the dragon can attempt to detect using a legendary action; that is, not on his turn. Ordinarily, an active wisdom (perception) check can only be made on your turn by using an action. \$\endgroup\$ – LegendaryDude May 19 '16 at 17:28
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    \$\begingroup\$ In other words, a dragon is ever-vigilant - which is actually one of the many meanings of "draconic." \$\endgroup\$ – GMJoe May 20 '16 at 1:28
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The dragon can find hidden PCs

If a PC hides, their Stealth roll goes against the NPCs' passive perception. If they beat it, they're hidden. At that point, the NPC can make an active perception check, rolling perception against what the PC rolled for their Stealth check. If they beat it, the PC is no longer hidden.

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  • \$\begingroup\$ Changed the wording to roll. I'm not where you get the idea that you use the passive stealth after that. Do you have a source? \$\endgroup\$ – Kyle W May 19 '16 at 17:26
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    \$\begingroup\$ Passive stealth? Never heard of it. It's a dex (stealth) check whenever you hide, contested against passive perception. You retain the result of your stealth check until conditions change and you need to hide again. There can be a separate active wis (perception) check made by the "target" against the already determined stealth roll of the stealthy character. \$\endgroup\$ – LegendaryDude May 19 '16 at 17:30
  • \$\begingroup\$ @SirTechSpec For stealth, you always use your initial dex (stealth) roll result until conditions change (you move to a new position, you are spotted, etc). \$\endgroup\$ – LegendaryDude May 19 '16 at 17:31
  • \$\begingroup\$ @SirTechSpec even the book mentions making a stealth check for NPCs when they hide. \$\endgroup\$ – Kyle W May 19 '16 at 17:38
  • \$\begingroup\$ OK, maybe I'm misremembering something my DM said about not having active opposed rolls. \$\endgroup\$ – SirTechSpec May 19 '16 at 17:40

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