A Spectator (a lesser Beholder, described on Monster Manual p. 30) can use its reaction to use its Spell Reflection ability:
Spell Reflection. If the spectator makes a successful saving throw against a spell, or a spell attack misses it, the spectator can choose another creature (including the spellcaster) it can see within 30 feet of it. The spell targets the chosen creature instead of the spectator. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.
My question is that if a ranger uses the spell Hail of Thorns and then hits the Spectator with an arrow, then if the Spectator makes its saving throw, can it redirect the thorns to another player?
The reason I question this is that the spell says that the thorns sprout from the ranged weapon or ammunition. In this sense, it seems that a person would have to have been hit by the arrow. However, other than this, the details of the Spell Reflection ability also suggest that it may be possible to make the thorns hit someone else even though it would still take the arrow damage.
Best Answer
No, because the Spectator has to save against the Spell itself, and the Thorns are not a spell in and of themselves
The spell Hail of Thorns does the following:
In this case, the spell doesn't actually target the Spectator, it buffs you. The enemy being shot is not the target of the spell and is not saving against the spell, it is saving against something the spell has created: essentially a bomb made of thorns.
For example, if a Caster launched Fireball at a Spectator and the Spectator succeeded, that would count as saving against a spell. But if Fireball was cast on the unstable cave ceiling above the Spectator, the Spectator might still make a saving throw, but if it succeeded, it couldn't retarget the rocks.
For another example, if a Caster attempted to Polymorph the Spectator, the Spectator could attempt to retarget the Polymorph if it succeeded on the Saving Throw. If the Caster Polymorphed an Ally into a creature, and that ally-creature missed the Spectator with an attack, the Spectator couldn't retarget Polymorph.
In this case, Hail of Thorns could not be retargeted because the Spectator is not saving against a spell, it is saving against a bomb which was made by a spell but is not itself a spell.
Edit: A Tweet from Crawford supports the idea that Hail of Thorns targets the Self and not the Spectator:
https://twitter.com/jeremyecrawford/status/606193562317766656