What is Uttara Kaula?

Uttara Kaula is sometimes translated as Northern Clan or Northern Family. The Northern/Uttara part comes from a fourfold division ( also sometimes five, six or seven, there are many other classifications mentioned throughout the tantras, so-called Krāntas or 'steps') or "transmissions" (āmnāya) of Kaula into Purva

Kaulism developed from Yogini cults, which in turn were derivied from the Skull baring Kāpālikas, derived from kapāla meaning "skull", and Kāpālikas means the "skull-men". The Kāpālikas traditionally carried a skull-topped trident (khatvanga) and an empty skull  as a begging bowl.(1)

The may have been in around as early as the 5th Century as renowned tantric scholar , Mark Dyczkowski has stated.(2)

One of the earliest references to a Kāpālika is found in Hāla's Prakrit poem, the Gāthāsaptaśati (third to fifth century A.D.) in a verse in which the poet describes a young female Kāpālikā who besmears herself with ashes from the funeral pyre of her lover. Varāhamihira (c500-575) refer more than once to the Kāpālikas thus clearly establishing their existence in the sixth century. Indeed, from this time onwards references to Kāpālika ascetics become fairly commonplace in Sanskrit ..

Notes:
1.Flood, Gavin  (2008). The Blackwell Companion to Hinduism. John Wiley & Sons. pp. 212–213. ISBN 978-0-470-99868-7.
2. Dyczkowski, Mark S. G. (1988). The canon of the Śaivāgama and The Kubjikā Tantras of the western Kaula tradition. SUNY series in Kashmir Śaivism. SUNY Press.