Shadow Radar
The Shadow Radar was introduced on 1st July 2019 to celebrate PokéHeroes 6th anniversary, in memory of the Pokeradar, back then called the Shiny Radar. The Shadow Radar is a permanent feature.
How it works
Our local computer scientist and favorite nerd in town, Bill, has developed a new device, called the Shadow Radar 
. This item makes it possible to find these Shadow Pokémon in the wild. It is available in the Item Shop after reaching Tainerlevel 20. You can either buy it with PokéDollars or with Nuggets.
After purchasing the Shadow Radar you will be send out on a 10x10 field of tall grass. Pokémon-Shadows 
will randomly appear that have to be clicked on.
If it turns out to be a regular, non-shadow Pokémon, it will start a chain of this species and count up in your Shadow Radar. The longer your chain, the higher will be the chance to fine a Shadow version of it. However, if you find a species that is different to your chain, your chain breaks and you loos the game.
Tip: Remember the field where you had you last Pokémon fight. The longer the distance between this field and the field of your next fight, the higher the chance of finding the same Pokémon-Species. You don't have to click on every appearing shadow. Sometimes it's better to wait a bit until it appears in a spot that is further away from the spot where you challenged the last Pokémon.
Also make sure to have enough Poké Balls 
with you! They are available at the Item Shop.
Some Pokémon are harder to catch than others and take a few more times. And not to mention that there's also a slim chance of the Shadow Pokémon fleeing during the fight, so maybe also bring some Ultra Balls!