What is a fitting reward for a hard encounter?

In my campaign, there is an optional enemy gauntlet filled with many myconids and fungi, meant to whittle down the players until the final round, where the entire grove gets re-animated and attacks as one entity. While I have the enemy gauntlet sorted out (Its basically a regular enemy gauntlet but the enemy, aka the "Grove" share HP and initiative),  I'm unsure about the reward. I was thinking a magic staff that when used, would allow the user to banish a chosen foe to a demiplane of harmless mushrooms (Flavour-wise, giant fungi would come out of the ground and swallow the target into the earth lol) for 1 Minute. This is similar to the Banishment spell, and the aim is for it to be a Rare Item. I would love to hear thoughts, con-crit, and other advice.