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?
Per the Way of the Open Hand monk's Open Hand Technique feature:
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.