As an addition to our Visions of N’Zoth Content Update Notes, here’s an outline of what’s changing over the next three weeks, and when:

With scheduled maintenance on January 14:

• Artifact Knowledge will increase, bringing a player-character with a level 70 Heart of Azeroth to 17500 AP needed to reach 71.
• N’Zoth Assaults and Horrific Visions begin, and items from those activities can be Corrupted, but will never Warforge/Titanforge.
  • Items from N’Zoth Assaults will scale with your character level, similar to other World Quests, and are already available at Season 4 item levels.
  • Items from Horrific Visions improve as you complete additional objectives, going from item level 420 up to item level 470 from a full clear with 5 masks active.
• All Titan Residuum will be converted into Silver.
• Titan Residuum will not drop from any source for one week. This includes scrapping or disenchanting Seasonal gear.
• In competitive PvP, Season 3 is closed for titles and Seasonal rewards. The Notorious Gladiator’s Proto-Drake is no longer available, however, the Vicious War Basilisk mount can still be completed this week.
• Mythic Keystone dungeons are still playable using Season 3 affixes, and their associated rewards remain available through this week.

With scheduled maintenance on January 21:

• The new Ny’alotha raid opens for Normal and Heroic difficulties.
• Season 4 begins.
  • Item levels increase by 30 for rewards from seasonal activities (dungeons, PvP, World Quests), and can now be corrupted by N’Zoth.
  • Mythic Keystones will downgrade by 4 levels.
  • Dungeon difficulty is increased for Heroic and Mythic difficulties.
  • During this week, end-of-run rewards from Mythic Keystone dungeons and Rated PvP are capped at item level 445 (for Mythic +6 or PvP Rank 2).
  • Mythic Keystone and PvP chests that are opened this week reflect Season 3 rewards earned the prior week, and cannot be Corrupted, but may Warforge/Titanforge.
  • Item level improvements for weekly rewards now extend to Mythic +15, which awards an item level 475 piece of gear in the weekly chest to be opened on January 28 or later.
• The Darkshore Warfront now has a Heroic difficulty, with item level 460 gear rewards.
  • Please note: Heroic difficulty will become available in the first Warfront that begins in each region after the Season 4 transition in that region.
• Any items not updated for Season 4 (for example, items from The Eternal Palace or Heroic difficulty Arathi Warfront) can continue to Warforge/Titanforge up to item level 455, but cannot be Corrupted.
• Titan Residuum is again available.
  • All items that can be purchased with Titan Residuum, including new Season 4 items, have had their costs adjusted.
• A Mythic +10 weekly chest will award 1700 Titan Residuum, with each additional level awarding 90 more Titan Residuum. For example, a Mythic + 15 will award 2150 Titan Residuum.
• Specific item level 475 tokens cost 20,000 Titan Residuum. Random item level 475 tokens cost 4750 Titan Residuum.

With scheduled maintenance on January 28:

• Ny’alotha opens for Mythic difficulty and the first wing of Raid Finder difficulty.
• Mythic Keystone dungeons at +7 or higher, as well as PvP Rank 2 or higher, now award end-of-run items that scale up to item level 465 at Mythic +15 and Gladiator, respectively.
• Weekly chests for Mythic Keystone and PvP activities now contain Season 4 items, which are always Corrupted, and scale up to item level 475. Again, these items will never Warforge/Titanforge.

For everything else you need to know about the Visions of N’Zoth content update, check out our full coverage in the World of Warcraft blog.

After Tuesday, it will no longer be possible to restore those treasure maps if they were deleted.

I’ve added the following to the OP here:

On January 21, with the beginning of Season 4,

• Mythic Keystones will downgrade by 4 levels.

Trying to answer this with great specificity:

After the content update (in this region, on Tuesday, January 14 starting at 7:00 a.m. PST), we remain in Season 3 for one week. So any Mythic Keystone rewards chests that you’ll open any time before the weekly reset on Tuesday, January 28 (in this region) will be Season 3 chests. They will not contain Titan Residuum.

Those chests will reward double the usual AP.

In Visions of N’Zoth, Titan Residuum is only obtainable from Season 4 chests and Season 4 Azerite items.