The Venomous Abyss is open: eight bosses, Ula'tek at the end, and the Race to World First is on

The eight-boss raid opened today on Normal, Heroic and Mythic alongside Midnight Season 2, and the race for the world's first Ula'tek kill is underway with Liquid, Echo and Method streaming.
The Venomous Abyss opened today in World of Warcraft: Midnight, eight bosses deep under the Coiled Isle and ending on Ula'tek. It was the missing piece: when Patch 12.1 went live on August 11 the island, the delves and player housing arrived, but the raid was held back until today.
Blizzard opened Normal, Heroic and Mythic at once, plus the first Raid Finder wing, which asks for item level 273. The rest rolls out weekly: Story Mode and wing 2 on August 25, wing 3 on September 1, and wing 4, the one with the Coiled Altar and Ula'tek, on September 8.
Mythic holds the expensive stuff: the Primeval Skyfriend mount drops from Ula'tek, and the Hall of Fame saves a slot for the first 200 guilds to kill her. That opening also started the Race to World First, with Liquid, Echo and Method streaming their pulls from day one.
Season 2 moves more than the raid on the same day: Mythic+ starts with eight dungeons, including the new Altar of Fangs, and Mythic 0 hands out item level 292. Classes went in with the tier set cuts Blizzard published over the weekend, so the race is being run on those numbers.