Speak with Sevri Il'Bethod who is standing next to the mailbox, outside the inn ( 2, 0, -36 ) Copy/waypoint 2, 0, -36.
Go to the grate at the north side of the fountain plaza ( 40, -4, -59 ) Copy/waypoint 40, -4, -59. When you approach this grate a dialogue window will appear.
Select "hold out the herbs and begin the chant".
Select the right words of the chant. They are: Kah, Shee-Jah, Ver-ata, Nik-Su, Gej-raz!
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