If you decide to save Olgierd von Everec's soul at the end of the main quest line, he'll give this as both his token of gratitude and departure from his old life.
The sword adopts a unique mechanic in combat. When the blade flashes red, using a strong attack can deal an incredibly powerful strike. However, it costs the user's vitality by 15% of their maximum vitality. The blade will flash red upon striking a target with a few fast attacks in a short period of time (around 3). Blocked strikes will also count towards this number. The charged strong strike will also bypass blocks and has a strong knockdown effect similar to an Aard Sign of intense power.
Iris hits twice if any kind of strong attack is executed regardless whether the sword has been charged, including Rend. Thus, a player who tries to use Iris should use strong attacks as their primary damage dealing tool. In addition, vitality is not lost as long as the sword is uncharged (read: glowing red); this means that the benefit of increased damage can be enjoyed as long as the sword remains uncharged.
- The sword's scabbard that Olgierd used to sheath the blade is somehow absent in the scene where he parts way with Geralt.