Before I begin this article, I need to acknowledge that the information came from Chrales and Kelven91.
Without dedicated and hardworking Pokémon GO dataminers, no one would ever have such information way in advance.
Initially found in the gamemaster as "NOEVOLVE" (6 days ago, as of writing), it appears that it has now been changed to "SHADOW" and "PURIFIED".
Though notably, Wurmple is now missing from the Shadow/Purified list.
Images from Kelven91
While the concept of Shadow Pokémon and Purification is reminiscent of mechanics from Pokémon Colosseum and Pokémon XD, the one main change from there appears to be that the Shadow Pokémon seems to be the result of Team Rocket's invasion of Pokéstops.
Along the same vein, Purification appears to be a Quest.
Images sourced from Chrales
Now, some trivia and a little snippet of history:
1. Shadow Pokémon (Dark Pokémon in Japanese) were introduced in Pokémon Colosseum, and the mechanic was also used in Pokémon XD: Gale of Darkness.
These Pokémon "shut the doors to their hearts", becoming soulless fighting machines.
They cannot: level up, evolve, be nicknamed, or receive changes to it's moves.
Some of their moves are replaced by Shadow Moves, and purification will replace the Shadow moves with standard movesets.
2. Pokémon TCG had a sub-class of Pokémon that turned "evil" when utilized by Team Rocket, and they were known as Dark Pokémon, not to be confused with Dark type Pokémon. Though in a later revival of the TCG mechanic, Dark Pokémon functioned as a dual-type, with Dark type as their secondary typing (if they already had a non-colorless primary typing).
No 'purification' mechanic existed for Dark Pokémon, though.
Image from bulbapedia.
3. Pokkén Tournament featured a Shadow Mewtwo (Dark Mewtwo in Japanese). It's Shadow status was due to the influence of the Shadow Synergy Stone that merged with it. It was subsequently restored to normal when the stone was separated from it.
Image sourced from bulbapedia.
4. In more recent history, Team Rocket could be seen photobombing players in Chicago's Pokémon GO Fest 2019.
Maybe it was a tease for what is to come?
Image sourced from Pokemongolive.com
As a last note of speculation: Team Rocket was behind the creation of the first Mewtwo (and it's armored variant).
Could this event be a huge tie-in with Mewtwo Strikes Back EVOLUTION, and also the way they introduce the elusive unused Mewtwo A seen in the gamemaster a few months back?
In any case, I am excited about this. Play safely and enjoy the upcoming Pokémon GO event!