The Great Ungez is a cockroach that can reach a size of up to four feet in length and approximately 30 pounds.
The Great Ungez is a megafauna roach and has stats fitting with being a monstrous insect. They are quick, can fly but poorly, have high hide and sneak scores, and are scavengers and thieves.
Cause Fear - as per the spell, people are terrified of giant insects, and roaches are particularly unclean and loathsome. The larger the number of ungez present, the stronger the fear effect becomes. This effect, in the case of a super swarm, is enough that even normally fearless things decide to take their business elsewhere.
There are two potential origins of the Great Ungez, divine curse or the typical answer. In the typical answer, ungez, roaches, have been exposed to magic, a forgotten valley, a growth potion, or any of the other dozen reasons that are created to justify mega insects. The second is more thematic and interesting.
Ungez was a man, once. He was considered a queer and dislikeable man, with an equally queer and dislikeable family. His father was noted for being a cruel man, and his sister was known for being heartless and controlling. They were all very much shut ins, and they spoke of heresy and blasphemy. This mortal vitriol was doled out to the gods of darkness, and other less savory things. While many though Ungez and his clan of being worshippers of the Dark, they were blasphemers of it, those who mocked the dark and the scuttling things and belittled things. Thus is was that a guest came to Ungez and asked hospitality, and made the sign of the Dark Ones. Ungez took the guest in, but they were shown cruelty and hostility. Satisfied with this response, the Dark Ones caused a venomous fume to fill the house of Ungez, a place riddled with rot and trash, and tiny crawling things, and Ungez and the rest of his kin were all turned into roaches of monstrous size.
Eldritch Ungez - while normal great ungez can be clever, Eldritch ungez are truly intelligent, and can speak in a scritchy buzzing approximation of the common tongue, most are better at writing. They allegedly have their own glypic language, worship the gods of rot and decay, and one in a hundred has clerical abilities.
Immortal Ungez - there is a story that Ungez himself still lives, and is indeed immortal, sentenced to live a giant roach, eating feces and trash until such time as the world ends. He is protected from death, and insanity, he is not allowed any escape from his roachy prison.
The Great Nest - there is a massive invae nest of ungez somewhere, full of intelligent ungez, and roach queens, and that one day this nest will burst and it will fill all of the underdark with billions of ungez.
Ungez Mage - a stripe of the eldritch ungez, the ungez mage has been exposed to magic radiation, and as such has gained innate magical powers, typically a single low level spell. If these ungez were to continue aging without being killed, it is possible that they could gain more spell like powers.