Some questions

[Ryuto]

Is this just a copypaste of the Halloween event?

What's so special about alpha pokemon? Just having their HA unlocked?

[JothonianTinku]

its similar to halloween but with way more npc encounters that always give you the equivalent of the goodie bag but without having to get candy first. so you get more rewards compared to halloween and the boss doesn't have 3 phases nor difficulty options, and it may be challenging depending if you have the right pokemon (if you do it might way easier than the pumpking). 
in terms of the alphas, yeah the only thing is that they have the hidden ability(only the kanto starters have alpha forms rn) and as bigger in the overworld compared to the normal variants. they do get some good ivs (maybe even perfect in some cases). but to get them you beat this events boss (will require a anniversary medal obtained from chests given to you by gym leaders, e4 and champion of the region you are(this ones are scattered around the overworld, they are not the actual npc you find inside the gyms))