Regarding the Slayer
Regarding the Slayer is a book given to the player by Volo after he is rescued from the heapside docks, reading it grants all characters a permanent passive buff that gives advantage on saving throws against Frightened inflicted by anyone in the Slayer form.
Where to find
Given by Volo after he joins your camp in Act 3, after rescuing him (again).
Even though this child of Bhaal has thus far been squandering the gift of their unholy blood, it seems that Bhaal's lash is inescapable. The graven God now has besiged them with the Slayer, the beast and prophet. And when they turn, their mind is broken, more broken than all my torments could ever rend it to. I spent all this while testing the captive Spawn with my experiments, trying to extract more of Bhaal's gifts from them, but they have brought the Slayer upon themselves.
It seems that it manifests in moments of pure hatred, a divine hatred those of mortal blood cannot fathom. When the child of Bhaal hates in a way that pleases their Lord, Bhaal himself descends upon them as his own avatar. Despite all the Bhaalspawn of Candlekeep has done to prevent Sarevok's uprising, it seems Father must love them dearly indeed to intervene personally on their behalf.
Perhaps if I continue my routine of absorbing all of the Divine Essence I extracted while the Spawn was my captive, I can force Bhaal's magics to quicken in my own blood, trick the fool God into thinking I'm one of his own children. Thus may I tame the beast for myself.
The book then continues with a detailed record of Irenicus' experimental attempts to transform into the Slayer.