The Severed Threads Nerub-ar Palace buff is now live and players can receive bonus damage and healing.
After you collect 16 Nerub-ar Finery, you can turn them into Anub-azal at the start of the raid. When you do, you get 75 Kej as your reward in addition to an update on the buff.
Unfortunately, the tooltip doesn't update and looks like it's bugged remaining at 0%, however the damage and healing buff does appear to be working as intended.
From testing, various spells inside the raid and outside the raid...
The datamined 3% seems to have been incorrect and the expected 2.25% from the original Blue Post was correct. This means that the buff will cap out at 18% on the week of January 21st.
Remember that Nerub-ar Finery can be obtained once per boss, per Warband per week (unless things changed this week. Mythic bosses will drop 2 Nerub-ar Finery per two (granting 1 additional for players that have killed a lower difficulty).