The quick answer

A Harbinger kill can affect eligible equipment and Forgotten Knights progress. Corruption is a separate system. Do not interpret “upgrade when you slay a Harbinger” as a character level-up or an upgrade to every item you own.

Which equipment upgrades?

Inspect the weapon or off-hand’s implicit text for “Chance to Upgrade when you slay a Harbinger.” It must be equipped when the Harbinger dies. A copy in your bag or stash does not satisfy that requirement.

For example, Kolheim Axe → Avenged Kolheim Axe gains an implicit bonus against frozen enemies. Cavalry Sword and Chitin Club have their own corresponding Avenged bases. Check the exact base on your item; this is not proof that every normal affix increases by one tier on every kill.

Circle of Fortune rank 6 can reward Rare Harbinger’s Spoils, while rank 10 upgrades that reward type to Exalted. Reward quality is distinct from the equipped item’s base conversion.

Make a Harbinger appear

  1. Follow Monolith progression: obtain 5 Knowledge of Orobyss, complete the required dialogue and take your first level-90 timeline Boss Quest. Defeat the timeline boss and its first Harbinger, then continue the Forgotten Knights quests. An ordinary Echo is not the Boss Quest.
  2. Press Y for Factions, choose Endgame Factions, then Harbinger Progress on Forgotten Knights. In its main window choose Harbingers. The developer’s FAQ says this window shows the minimum Corruption needed for a Harbinger to spawn.
  3. Read your current target and Corruption requirements, build the needed Stability in the appropriate timeline and prepare its Boss Quest. Ensure Harbinger Crest is checked: the official FAQ explicitly says it will not spawn a Harbinger otherwise. Defeat the timeline boss, then the Harbinger.
  4. Complete the Forgotten Knight’s quests and defeat stronger Harbingers to raise faction rank. Follow the next target; repeating an already completed target does not by itself complete a different quest.

Confirm the right result

Check the equipment’s exact Avenged base, the faction quest/rank and the timeline’s actual Corruption separately. Shade of Orobyss remains the Corruption progression route. Faction benefits help you climb faster, but they do not establish one fixed Corruption increase for every Harbinger kill.