Those that enter the resting place of the Titan rarely emerge. But those that do are permanently imbued with some of its power, relinquishing their names as part of this new communion. While I've tried to wring an account from them on their time with the Titan, their minds seem to have expelled any contact with this Unnamed power.
Furystone is effective on units who already have a way of gaining Rage because of the way factorials mathematically progress/regress (pseudo-exponentially, with Rage decay/regressing at a rate of 1 per wave).
Runestone is effective on units who have the Incant affix. You will already want to regularly cast your spells on a particular floor (with your Incant units). Having Runestone on a unit (especially front line units) with Incant will gain an additional bonus. Though Nameless Siren isn't a front line unit, during the Relentless phases of the boss fight, the Armor provided by Runestone still allows her to attack more times and deal more damage.
Frenzystone is effective on units with the potential to obtain a high Attack value, effectively double the damage dealt (which could mean the death of two enemies, rather than one). Doubling a value of 100 Attack is far more effective than doubling 10 Attack.