Tiamat can take a Legendary Actions at the end of each turn, and I believe these were listed as reactions. Could an Open Hand Monk's Open Hand Technique prevent these legendary actions entirely?
Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on that target:
- It can’t take reactions until the end of your next turn.
If that is possible, that interpretation seems overpowered - unless the Legendary Actions are NOT reactions (I may be remembering this incorrectly), or the reactions are treated differently because they're Legendary Actions rather than normal Reactions.