To complete the PoE 3.28 challenge of obtaining a level 21 or 23% quality gem in Path of Exile, you must use a Vaal Orb on a level 20, 20% quality gem. The most efficient method for a 21/20 or 20/23 gem is using a Vaal Orb; for a 21/23 gem, use the Locus of Corruption (T3 Gem Room) in Alva’s Temple.

Method 1: Using a Vaal Orb (Single Corruption)

This is the most common method but only allows for one positive outcome.

Target Gem: Use a Level 20 gem (to hit Level 21) or a 20% Quality gem (to hit 23% Quality).

Possible Outcomes:

• Level Change: Add or subtract one level. If your gem is Level 20, it has a chance to become Level 21.
• Quality Change: Add or subtract 1-10 quality. If your gem is 20% quality, it has a chance to reach a maximum of 23% Quality.
• Vaal Version: The gem may transform into its Vaal counterpart (if one exists).
• No Effect: The gem becomes "Corrupted" with no stat changes.

Method 2: Doryani's Institute (Double Corruption)

To increase your chances or attempt to get both 21/23 at once, use the Lapidary Lens in the Level 3 Gemcutter's Room (Doryani's Institute) in the Temple of Atzoatl.

• Effect: It applies two corruption effects simultaneously.
• Benefit: This is the only way to reliably aim for a 21/23 gem. It can also result in a Level 21 Vaal Gem, which is not possible with a single Vaal Orb.

Doryani's Institute is the tier 3 incursion room of Zalatl, Architect of Thaumaturgy.

The room contains a device called Lapidary Lens that "double corrupts" a skill or a support gem. The room also contains Thaumaturgy Vessels containing gems and Gemcutter's Prisms.

It is capable of creating, and the only way (other than divination cards) to acquire gems that are two of the following:

• Level 21 (or otherwise 1 level higher than its normal maximum level for Awakened Support Gems/irregular gems)
• (21-23)% Quality
• Vaal version