billd91
Not your screen monkey (he/him) 🇺🇦🇵🇸🏳️⚧️
You could, conceivably, use it on a reaction on your own turn such as in the example you lay out. But horde breaker is explicit in that it only applies once during your turn. So if you used it during an attack action, you can't use it during your reaction later in your turn. Or, if you used it in a reaction early in your turn, you can't then use it during your attack action.But if you use your action to cast a spell, and an enemy uses their reaction to run on your turn
You could hit with a reaction on your turn, and then horde breaker.
That's a lot of If's. But technically possible.
Nutshell version: In other words, it doesn't matter how you get that attack that triggers horde breaker - attack action or reaction that involves an attack - you only use horde breaker once on your turn.