The haste spell, as well as the Potion of Speed which mimics it, grants an additional attack on each of the creature's turns. The wording says "one weapon attack only", but…
There's a 3rd-level Hunter ranger feature called Horde Breaker that allows the ranger to make one additional attack (with the same weapon, against another target) when he makes a weapon attack. So…
Let's say a ranger drinks a Potion of Speed as his action, then attacks with the additional attack from haste. Would he then be able to benefit from Horde Breaker and make an additional attack?
Best Answer
Yes, you can use horde breaker with the Attack granted by haste
Horde Breaker says:
Since haste grants the ability to make one weapon attack as part of an Attack action, it will definitely be a valid trigger for the Horde Breaker ability.
This will work even though haste specifies that the Attack action can be used to take only one weapon attack, because the Horde Breaker attack is not made as part of that Attack action. It just happens to be triggered by that action, so it does not violate that stipulation.
So, because the haste attack is a weapon attack, you can use Horde Breaker to make an additional attack.
A couple things to note though: