Use your Death Cultist Disguise to enter the Death Cultist Base Camp, then put Betina's mixture into the Mereldar Plague Cauldron.
- Disturb Mereldar Plague Cauldron
- Death Cultist Disguise (Provided)
Now, mix a tiny amount of rot into the water, and mix thoroughly. There, that's it. This solution will still be very powerful, and we only need a little bit of it.
On the lake's southeastern border, you will find a plague cauldron with several cultists nearby. Put on your cultist disguise, then approach the cauldron and put in ONE DROP of the solution you made. No more, or you may risk your very life!