The Pit of Artificers - New Endgame Content Coming to Diablo 4 Season 4

wowhead.com:

Season 4 will be Diablo's biggest Season yet! In the recently posted PTR Patch Notes, Blizzard revealed more details about the new endgame activity coming to Season 4 — The Pit of Artificers. This new challenge will become available once players complete a priority quest after entering World Tier 4.

The Pit of Artificers will contain hundreds of levels of difficulty and will provide players with Masterworking materials to aid them in the newest endgame crafting system!

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Blizzard

The Pit of Artificers

• Upon entering World Tier IV, you’ll receive a priority Quest to complete a Tier 46 Nightmare Dungeon. Upon completion, you’ll begin to collect Runeshard during endgame activities. Collect enough Runeshard to ultimately activate the Obelisk in Cerrigar.
• Activating the Obelisk will send you to The Pit. Your goal will be to slay enough monsters before a 10-minute timer runs out. Each player death removes time from the clock: first death removes 30 seconds, second death removes 60 seconds, third and subsequent deaths will remove 90 seconds.

• If you slay enough monsters, a portal will appear, taking you to the boss’s arena.
• Killing the boss before the timer expires will grant you Masterworking materials and unlocks the next tier of The Pit.
  
  Killing the boss with 4-6 minutes to spare grants you an additional Tier unlock.
• Killing the boss with 6 or more minutes to spare grants you two additional Tier unlocks.
• The Pit has hundreds of Tiers of difficulty.
  
  If the Mastery timer expires before you finish running through the Pit you’ll still receive loot, but won’t receive any Masterworking materials.