"Falling leaves, red and brown and black and gold. A rain of burning machine over a city of the Apt. The darkness between trees. The seal of the worm is breaking." Quote from Cheerwell Maker From Heirs Of the Blade
An ancient kinden that in the Bad Old Days were banished.
(Spoilers) Possible Worm Kinden ArtEdit
It is implied that the Worm kinden posseses an art which prevents apt thinking as shown in the ending of War master's Gate.
Worm kinden burst forth from the ground in the middle of a Wasp encampment. The wasps try to rally and fight back, but find themselves filled with a feeling of primal terror and being unable to use any sort of machinery or mechanized weaponry. From this, It's clear that the Worm Kinden are exerting some sort of "apt-dampening" aura.
If this is an art of theirs or a spell of some sort has yet to be confirmed.