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Zaith'isk
A zaith'isk is a partly organic, partly mechanical Githyanki device that purportedly purifies a Mind Flayer Tadpole from an infected victim. One can be found in the Infirmary of Crèche Y'llek. You can speak with Ghustil Stornugoss, the resident doctor, to begin the procedure.
Usage
When you first talk to the Ghustil, selecting I'd prefer Lae'zel to speak on my behalf gives you 
+1 
-1.
If Lae'zel is with you, you gain +5 and she will insist on using the machine first. If you select Stand back. I'm going first, followed by You would still be hunting for this place without my guidance. I will be cured first, you gain +3. Alternatively, allowing her to be purified will gain you +10. If you instead select Make way, Lae'zel. I am purging myself of this damned parasite and subsequently commit to that decision, she will turn hostile against you along with the Ghustil.
You will need to pass three saving throws in order to successfully complete the procedure.
- DC 12
Intelligence saving throw - DC 15 Wisdom saving throw
- DC 18 Constitution saving throw or DC 18 Intelligence saving throw
Several classes have an additional choice for the third save, letting them use their main ability stat to more easily overcome the DC 18.
After the third step of the procedure, if you are a cleric you can select Pray for deliverance, Flood the creature with divine power for +1. Finishing the procedure gains you +8.
If Lae'zel is the one in the machine, you will not see any saving throw screens. It is possible that the persuasion/deception rolls on behalf of the player take the place of the saving throws. Choosing to tell her to remain in the machine each turn seems to cause her to automatically take the corresponding penalty. Regardless, you will gain +1.
Secrets of the Zaith'isk
If you pick the option Examine the device and pass a 
DC 15 Investigation check:
- *The device is an ingenious synthesis of illithid anatomy and metal alloys. It hums with psionic energy, hinting at paths into unknown minds and unseen planes.*
If Lae'zel is convinced to leave the machine at any stage, be it by trickery or persuasion, you gain 
+1 +1 +1 
+1 
+1 
+1 
+1.
Stage one
You can persuade Lae'zel to jump out of the machine as soon as the procedure starts with a DC 30 Persuasion check. Alternatively, you can let the procedure continue and attempt a DC 30 Deception check to trick her out of the machine. A passive Perception check will tell you:
- The zaith'isk's psionic forces batter Lae'zel's thoughts. There is no chance she will survive this unscathed.
Telling Lae'zel to endure (Endure, Lae'zel. The cure is worth the torment.) gives +1.
If you are in the machine and pass a passive Perception check:
- *You realize the device is hunting your parasite. But it's doing so blindly. Without direction, your faculties could be permanently damaged.*
Stage two
If Lae'zel is in the machine and you pick the option *Seek answers within the zaith'isk* and pass a DC 21 Wisdom check:
- Lae'zel's life is laid bare among those that came before. Her thoughts are turned to silver thread and relayed to the Astral Sea.
- The voices of the dead cry out as one. The zaith'isk collects memories from the infected - and executes them. It is not for curing. It's for killing.
This decreases the difficulty of the Persuasion option to have Lae'zel leave the machine from 30 to 21. Succeeding will free Lae'zel and give +1.
Alternatively, telling her to endure (Keep focused, Lae'zel. The cure is close.) gives +1.
Stage three
If Lae'zel is in the machine, you can attempt to persuade her to leave again. This difficulty will either be 21 or 30, depending on the previous choices. Attempting to deceive her remains a DC 30 Deception check.
Alternatively, a DC 30 Wisdom check or a DC 30 Arcana check will allow you to force Lae'zel out of the device. The latter will also grant you the Awakened buff.
If you are an in the machine and succeed on any option, you will gain +3.
Outcome
Each failed saving throw will, respectively, give -2 to Intelligence, Wisdom, and Constitution. Consuming a 
Mind Flayer Parasite Specimen with the affected character will undo these debuffs.
If you succeed all three saving throws, your parasite will strengthen and you will gain the Awakened buff, which allows you to use your Illithid powers as a bonus action instead of a regular action.
Regardless of whether you succeed or fail, the zaith'isk will be destroyed by the Dream Visitor, who may exclaim, "Enough!".
If you pass a DC 15 Deception check or a DC 15 Performance check or if you were in the machine, Ghustil Stornugoss will let you leave. Otherwise, she will fetch assistants, locking the door ( DC 10 Sleight of Hand check) behind her. After some time passes, some Githyanki will barge in and try to kill you. This will not cause the others in the creche to turn hostile, and you can safely teleport to the nearby Sigil to avoid the fight.
External links
Zaith'isk on the Forgotten Realms Wiki