Generation 4 has finally been introduced to Pokemon Go, after many months of waiting. We all have been waiting for the Gen 4 Legendaries, and the first one to come was Giratina followed by Cresselia.
This Pokemon (Cresselia) is the Tier 5 Raid Boss at moment. Our suspicions are that the next Raid Boss to be introduced is Darkrai. These two Pokemon are known as the Lunar Duo.
This is most likely because it is typical for the duos and trios to follow back and forth in the raid roster. For example, we can take the Legendary Titan trio: Regirock, Regice and Registeel, the Tower Duo: Lugia and Ho-Oh and also the Weather trio: Groudon, Kyogre and Rayquaza.
The base of the Pokemon called Cresselia is the crescent moon, and the base of Darkrai is the new moon. Because these two intertwine, it is most likely that when Cresselia leaves the Tier 5 Raid battles in the next legendary rotation in December, and the Mythical Pokemon Darkrai will take its place. The relationship between the Lunar Duo is quite close, more than the other legendary packs.
The way they behave with each other can be seen all through the Gen 4 Pokemon lore. In the main series, Darkrai was caught on Newmoon Island, while Cresselia was caught on Fullmoon Island. Cresselia is known for giving people nice and calming dreams, while on the other hand, Darkrai gives people nightmares.
Be aware that these are only theories of ours. We cannot wait for December 18 (when Cresselia leaves the Raid Battles), because that is the day when we will find out if Darkrai will follow Cresselia in the Raid Battles.