Legendary Raids are a thing. So far this one and Articuno. I'm 0 for 3 on the Articuno though.

I just got articuno today.

Mildread: They're easy. Just go in, Click on the raid pass (you should get one free one per day by spinning a gym), and then you have two minutes to select pokemon to go against the raid boss.

Once it starts, tap and dodge. You can solo a 1 tier and a 2 tier, but you need help for the rest. A Legendary takes about eight people, though we did it with seven today.