Dragonflight Season 1 arrives to World of Warcraft on December 13 and brings a new raid, Mythic+ dungeons, a new Arena, and a new PvP season.
The first raid players will experience in Dragonflight is the Vault of the Incarnates. It features eight new boss encounters and will open with the weekly maintenance for each region. The raid will be released in three wings, with the first wing opening on the week of December 19. Players will need to have at least 16 out of 20 raiders from the same guild and faction to be eligible for rewards.

Dragonflight Season 1 also introduces four new World Bosses and Primal Storms to the game. Players can collect Elemental Overflow from the Primal Storms and spend it with Mythressa in Valdrakken to earn a full set of armor for their class and specialization, as well as pets and mounts. Enemies in each storm also have a chance to drop their Primal Core, which can be redeemed for rewards.
Mythic+ Season 1 features a new dungeon rotation, with four dungeons from Dragonflight and four returning dungeons from previous expansions. There will be no loot caps for Mythic+ on the first week of the season, and the first affix will be Fortified. Players will also be able to earn rewards by completing Mythic+ dungeons, such as class armor sets and the Nokhudon Proving Grounds arena.

PVP Season 1 also brings new rewards, such as the Primalist Class armor sets and the new Nokhudon Proving Grounds arena. The arena features two teams of two players each, and players can earn rewards by completing matches and reaching certain ratings.
Full patch notes: https://news.blizzard.com/en-gb/world-of-warcraft/23892221/dragonflight-season-1-content-update-notes