The Grotto of Yun-Ne'Shel was a cavernous organic creature that was created by the Yuuzhan Vong.
These large organisms were distant relatives of the yammosk and were used by the shaper caste. Shaper Initiates often entered into the grotto where they were bathed in liquid after which they wore an oozhith. All experiences felt by the Shaper was intensified by the telepathic abilities of the grotto.
With that done, they moved to a point in the chamber which contained a circular hole which was actually a mouth of the creature. The shaper was required to put their hand into the mouth after which sharp teeth punctured the skin and grinded into the bone. Once it bit deep into the body, the mouth rotated and snapped the hand off the shaper.
The shaper then had to make their way into a pool within the grotto which contained a small pond that contained the organisms that become Shaper hands.