So I chose spore druid to kill enemies and raise corpses as reactions.
Halo of Spores is supposed to trigger when enemies enter its radius. I have only seen this occur ONCE during the harpy fight in grove while in the water, where a harpy literally came upto melee me.
It is also not present in the reactions tab.
However I can manually cast it on one person within range, but that's on my turn not when they enter. From what I understand this can occur per characters turn as we have all seen how paladin has made multiple smite reactions when attacking more than once (powerful and disgustingly broken).
As a side note, I have also noticed Polearm mastery not triggering when enemies enter my attack radius while wielding a quarterstaff, will make a separate post but just mentioning it here too.