New Raid Boss - Palkia
On January 28, 2019, Niantic announced the new raid boss to be Palkia starting the next day. The Spatial Pokemon will be available as a boss until February 28, 2019. See below for relevant guides and full announcement text.
Full Announcement Text
A new arrival to raids will test your strength in 2019! The Water- and Dragon-type Spatial Pokémon, Palkia, will join Raid Battles across the world from January 29 at 1:00 p.m. PST to February 28 at 1:00 p.m. PST!
With the ability to warp the very fabric of space and enough power to take on Giratina, Palkia is a Pokémon that shouldn’t be underestimated. Be sure to assemble a team that includes Dragon- and Fairy-type Pokémon to take down this Legendary Pokémon before its special attacks overwhelm your team!
Don’t miss this chance to catch this Legendary Pokémon before it returns to its own spatial dimension! Stay safe, and happy exploring!
—The Pokémon GO team