Well, now that the game’s one-year anniversary has rolled around, Niantic is finally answering that question through their worldwide event this Saturday, called Pokemon Go Fest.

Niantic announced via a blog post that the “mystery challenge” they previously hinted at was indeed about Legendary Pokemon. Below is a greakdown of how everything is going to work:

In Phase 1, players worldwide will catch as many Pokemon as possible in the three challenge windows: 11 AM-11:30 AM CDT 12 PM-12:30 PM CDT 3 PM-3:30 PM CDT If enough Pokemon are caught during Phase 1, Phase 2 will take place from 6 PM to 7 PM in Grant Park, Chicago, as Trainers there will battle the first-ever Legendary Pokemon, raid battle style. If they defeat the Legendary, Phase 3 will begin worldwide – simply, Legendaries will start appearing as raid battles.

Legendary raids will be distinguished by appearing beforehand as “legendary eggs”, presumably with the design shown in the trailer – a blue egg with a white swirl. These Pokemon will be so powerful that you won’t be able to deposit them in gyms at all (but they’ll probably be incredible for clearing them).

Niantic didn’t specify which Pokemon would be debuting, but the trailer showed off the full set of birds – Articuno, Zapdos, Moltres, Lugia, and even Ho-Oh. The end of the trailer also had a blink-and-you’ll-miss-it Mewtwo, so make of that what you will.

Think this is a great way to celebrate Pokemon Go’s one-year anniversary? Which Legendary Pokemon are you most excited to get your hands on? Let me know down in the comments!