A jumara, also known as a Worm, is a type of Shadowspawn that was created by Aginor sometime before or during the War of Power, and found in the Blight. They can live for thousands of years and get stronger if the One Power is used against them. The only source depicting any encounter with a jumara, juvenile or mature, is the River of Souls chapter that was cut from A Memory of Light.
Unlike other Shadowspawn, they don't seem to be marshalled by the forces of the Dark One against the forces of the Light. Demandred speculates that Aginor created them not as a means of achieving victory for the Shadow, but simply to create as horrific a creature as possible. They seem uncontrollable and the usual mechanism that allows the Chosen to command Shadowspawn does not apply to jumara, as evidenced by the fact Demandred is forced to battle one to the death.
They do not feel pain, only hunger and (sometimes) fear. In order to survive an encounter with a Worm, the Worm must be cut to pieces. They are therefore creatures of pure destruction and instinct. They prefer human meat over Trolloc meat.
Physical description and size
We have no physical description of the juvenile jumara. It's possibly that their nickname "Worms" indicates that they do not have legs and crawl or burrow, if extremely quickly.
Adult jumara are multi-pedal insectoid creatures. They are covered in spines and tentacles, particularly around their mouth, and grow to gigantic size. Direct use of the One Power against them serves only to make them stronger. They were noted by Demandred as being nearly immortal unless slain, as they could go into hibernation for millennia, waking only if prey happened to touch one of their tentacles or spines.
Juvenile Jumara (Worms)
Like many Shadowspawn, the natural habitat of jumara in the Third Age is the Blight. Unlike other Shadowspawn, they don't seem to be marshalled by the forces of the Dark One against the forces of the Light.
Sammael mentions that those found in the Blight are called "Worms" and are fully-grown, but will never "transform". The conditions to trigger the transformation are not known, and therefore neither can the reason this will never occur for those living in the Blight.
Worms are extremely dangerous. A single Worm can easily kill a Myrrdraal unless the Myrdraal is extremely lucky; most of the terrifying creatures in the Blight will flee from a Worm, although the Worms themselves are scared of whatever creatures live in the high passes of the Blasted Lands. They travel in groups known as Wormpacks and cry to one another with a reedy, shrill scream. The Emond's Fielders with Lan and Moiraine hear a group of half a dozen worms "singing among themselves" as in a chorus. They are fast and effective at overtaking their prey.  Fain (as Mordeth) kills a Worm and says that a pack of worms can kill a whole legion.
Adult jumara are vastly more dangerous than their juvenile forms.
While searching for Sakarnen in Shara, Demandred encountered an adult jumara at Rai'lair, The Hearttomb. Somehow the creature had found the ancient cavern as a juvenile, and had matured to adulthood inside. The resulting creature had been too large to fit back out through the mouth of the cave, and so it had made the cavern its home, eating anything that entered.
It was described by Demandred as being easily 100 feet long. Demandred only just barely managed to slay the creature, and afterwards commented to himself that had he not known exactly what it had been and how to go about killing it, he would not have survived the encounter.