We know that Kalabhairava was created from Siva's forehead.
"Mahadeva then created a wonderful person, Bhairava, from the middle of his brows to quell the pride of Brahma. - Siva Purana Vidyesvara Samhita 8.1
"O sage, on hearing the haughty words of the lotus-born Brahma, Siva became angry as if out for destruction.
Angrily, he created a Purusa Bhairava blazing in great brilliance. Then he spoke lovingly.
O Kalabhairava, at the outset this lotus-born Brahma shall be chastised by you. You shine like the god of death, hence you are Kalaraja.
You are called Bhairava because you are of terrifying features and you are capable of supporting the universe. Since even Kala is afraid of you, you are called Kalabhairava.
- Siva Purana, Satarudrasamhita, 8.44-47
Later, Kalabhairava chopped off Brahma's fifth head.
However, the same Siva Purana also describes Ashtabhairavas (eight Bhairavas, namely Asitāṅga, Ruru, Caṇḍa, Krodha, Unmatta, Kapālī, Bhīṣaṇa, Saṃhāra).
"O Virabhadra, O Nandin, O eight Bhairavas, the frontier guards, let the Ganas start along with my sons at my bidding. - Siva Purana, Rudresvara Samhita, Parvati Khanda, Section III 33.2-3
Later we can find eight different Tantras for each of these Ashtabhairvas.
My question is, what is the relation between Kalabhairava and Ashtabhairvas.