An Adult Red Dragon is within melee range of a fighter with the Sentinel feat.
On its turn, the dragon moves away, and the fighter takes an attack of opportunity, reducing the dragon's speed to 0.
From the Sentinel feat (PHB, page 169):
When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.
The fighter then takes his turn. At the end of the fighter's turn, the dragon takes the Wing Attack legendary action.
Wing Attack (Costs 2 Actions)
The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Am I right in assuming that the dragon wouldn't be able to move because its speed would be 0?
Best Answer
Sentinel only affects the dragon on the turn of the opportunity attack
The relevant part of the Sentinel feat (PHB p. 169) with my emphasis:
So when the fighter makes a opportunity attack, the dragon's speed is reduced until the end of the dragons turn. This means that on the fighters turn, when the legendary action happens, the dragons speed is reset. Notably however, the fighter would be free to make another opportunity attack† against the dragon if the Wing Attack would move the dragon out of the fighters reach, stopping it from flying away (on a hit).
† As the fighter as regained the use of their reaction with the start of their turn. PHB p. 190