The Whisperer queues a special attack when she reaches or falls below 720 (80%), 495 (55%) and 270 (30%) Hitpoints. During the special's animation she takes 70% less damage outside the Shadow Realm and 60% less within (thralls ignore this reduction). She runs one of two rotations each kill: Corrupted Seeds → Screech → Soul Siphon, or Screech → Soul Siphon → Corrupted Seeds.
Corrupted Seeds. She moves to the centre and scatters corrupted seeds around her. You must step on all the light-green seeds to stop the attack — and they are only visible in the Shadow Realm. Fail before the timer runs out and you take 75 damage and lose 75% sanity. Stopping it cleanly without touching a dark-green seed restores 15% sanity; stepping on a dark-green seed deals 15-25 damage. The efficient method is to flip into the Shadow Realm, memorise the light-green layout, then re-activate the blackstone fragment and step on them in the real world to save sanity and prayer.
Screech. She teleports to the southern end of the cathedral and gathers energy for three screeches, while floating columns rise from the water to block them. The pillars have 20, 40 or 60 health, visible only in the Shadow Realm. You must stand in the 2x2 square directly behind a pillar to be safe. Each screech deals 20 damage to every pillar and also destroys the one you hide behind, so you save the 60-health pillar for the final screech — hide behind a 20-health pillar, then a 40, then the 60. Taking a screech deals 45 damage and drains 45% sanity.
Soul Siphon. She appears in the centre and summons twelve lost souls, siphoning energy for a chant that completes in 20 ticks (12 seconds). You must enter the Shadow Realm and kill at least one full set of souls chanting the same phrase — souls can only be damaged in the Shadow Realm. Fail and the chant deals 50 damage to you and heals her 100 Hitpoints. This is why the venator bow is so valuable: it can clear all twelve souls in the time limit because its shot ricochets between souls. The set you kill determines the effect — yellow souls restore ~20% Hitpoints, blue restore ~20% Prayer, cyan restore 15% sanity, and green deal 50 damage to her (75 if all twelve are killed).
After a special she usually launches a binding projectile that always binds you for ~8 seconds, then moves in to melee. Any ice spell or standard-spellbook binding spell freezes her in place even if it misses, which cancels the bind. Standing at least 10 tiles away after she launches it gives you room to run, as she stops before reaching melee range.