The Sepulcher of the First Ones is the new Shadowlands raid coming in Patch 9.2.

The first 8 bosses of the raid will be available on March 1st for Normal and Heroic difficulty. On March 8th, all 11 bosses are scheduled to be available on Normal, Heroic and Mythic difficulty.

The Raid Finder wings open on a staggered basis:

Wing 1 - March 8th
Wing 2 - March 22nd
Wing 3 - April 5th
Wing 4 - April 19th.

Note that queuing for Raid Finder requires a minimum item level of 220.

Currently the gear drops are shown to be RF:239-246, Normal:252-259, Heroic:265-272 and Mythic:278-285. Only the last 3 of the 11 raid bosses drop the higher-level gear.

Here's a good article.

And here is a guide from Icy-Veins which includes some preliminary videos.