Shapechanger

The Chosen Descender can use its action to polymorph into a Small or Medium humanoid, or back into its true form. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.  

Ma's Whisper

The Chosen Descender constantly whispers to itself while in its true form. Creatures that start its turn within 20 feet of the Chosen Descender must succeed on a DC14 Wisdom saving throw. On failure, the creatures movement speed is halved until the start of its next turn.  

Variant Insanity rule.

If you're using Erenel's Insanity rules for 5E, make the following addition to the Chosen Descender. Ma's Whisper. When a creature fails the Wisdom saving throw they also gain 1 level of Insanity. Creatures affected in this way can only gain 1 level of Insanity from an individual Chosen Descender every 24 hours.