What item level gear you get from Horrific Visions of N'zoth in BFA Patch 8.3 

Horrific Visions with certain chance can reward gear piece for defeating Thrall or Alleria. The item level of this gear piece depends on how many other activities you did in the Horrific Vision, as well as how many Faceless Masks were active. You must complete the main vision objective to receive loot. You'll continue to earn gear each run, but at a decreasing item level tier if you've already done a Horrific Vision run that same week with the same amount of objectives and faceless masks active. Qualifying for a new higher item level tier will reward the appropriate item level gear and then resume where you left off. After earning all tiers of gear, you will no longer receive gear.

Condition Item Level Rewarded
Main objective only (Thrall or Alleria killed) 420
Main objective + any 2 zones 430
Full clear (main objective + all 4 zones) 445
Full clear + 1 Mask 450
Full clear + 2 Masks 455
Full clear + 3 Masks 460
Full clear + 4 Masks 465
Full clear + 5 Masks 470

Examples

No masks, you do a Horrific Vision run and clear 3 zones and kill the last boss. You earn a 430 item level piece.
You do another run that same week and clear 3 zones and kill the last boss again. You earn a 420 item level piece.
You try a 3rd time that same week and manage to finally do the full clear (4 zones and the main objective). You get a 445 item level piece.
You then try for a 4th time that same week and again clear 5 out of 5 objectives. You will receive no gear this time, since you have already earned the 445, 430, and 420 tiers this week.