Caution: All rooms in the area toggle CPK / LPK / NPK and are all dizzies. Also randomly toggles no-magic. Recommended fourth class + with a good amount of hp to withstrain aggro mobs in no-magic.
Fight the Nefarious Couatl in the hidden down in safe, until he makes you shout. Having someone heal you while fighting is a good idea.
It seems that there may be an undisclosed timer before the Mark is given again after someone has gotten it. Either that, or the Couatl has to be killed and repopped. The Couatl will still make you shout upon fighting him, but no mark will be given.
For displaying a disturbing propensity towards suggestion, the Powers have granted thee a Mark. You have been granted The Mark of Hypnosis! You gain 41 practices!