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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.
Involvement
When encountered, the party can talk to Ghustil Stornugoss, the resident doctor, to ask that the zaith'isk be used to remove the tadpole from the party member's head. Once the party lies down in the machine, they must pass three saving throws in order to successfully complete the procedure, an 
Intelligence saving throw, a Wisdom saving throw and a Constitution, Wisdom, Charisma, or Intelligence (DC 18) saving throw depending on the class.
Warning: If speaking to Ghustil Stornugoss and then killing her, then the party will be unable to interact with the zaith'isk. To use the machine the party must either interact with it before speaking with her or leave her alive until after using it, if speaking with her.
Outcome
The failed saving throws will, respectively, give -2 to 
, 
, and 
. Consuming a 
Mind Flayer Parasite Specimen with the affected character, either through the inventory or by unlocking an Illithid Power, will get rid of these debuffs. One specimen will get rid of all three debuffs; multiple are not needed.
If the party passes all three saving throws, the parasite will strengthen and give the 
buff, which allows using all Illithid powers as a bonus action instead of a regular action.
Regardless of whether succeeding or failing, the zaith'isk will be destroyed by the Dream Visitor, who may exclaim, "Enough!".
If passing a 
DC 15 Deception check (or a DC 15 Performance check if the player character was in the machine) Ghustil Stornugoss will let the party 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 the party. This will not cause the other characters in the creche to turn hostile, and it's possible to safely teleport to the nearby Sigil to avoid the fight.
Special interactions with Lae'zel
If Lae'zel is an active party member, the player character will gain 
+5 and she will insist on using the machine first. The party can opt to demand to go first by selecting Stand back. I'm going first, but Lae'zel will object. The party can convince her using a Persuasion or Investigation check. Succeeding a check grants +3 while failing results in -3.
Alternatively, the party can demand to go first, and if sticking to that decision it will result in Lae'zel turning hostile against the party along with the Ghustil. Otherwise, letting Lae'zel go first will grant +10.
The party can then go through a similar set of three stages as if the player went through the process, however it is slightly different. Instead, the party must convince Lae'zel not to go through the process after realizing the machine is intended to kill her. If Lae'zel is convinced to leave the machine at any stage, be it by trickery or persuasion, the player character will gain 
+1 +1 
+1 
+1 
+1.
Lastly, on the last stage, the party member can try to take control of the zaith'isk and absorb its power by passing a DC 30 Arcana check to gain the passive themselves.
Related quests
External links
Zaith'isk on the Forgotten Realms Wiki