Shiva's Eight

Bhavah sharvo rudrah pashupatih atha ugrah saha mahan
Tatha bhima ishanav iti yadabhidhanashtakam idam 
Amushmin pratyekam pravicharati deva shrutir api
Priyayasmai dhamne pranihita-namasyo asmi bhavate
"Bhava, Sarva, Rudra, Pashupati, also Ugra with Mahan, furthermore Bhima and Ishana, these are eight appellations of the Lord, each of them the shrutis individually expound. My salutations are to the dear abode in which these names are laid i.e. to One who bears these names." (Shiva Mahimna Stotra, Verse 28)
Shiva Purana also teaches us that the entire creation comprises eight forms of Shiva who reside and pervade in the Universe like pearls on a string. Here are the names:

 Sarva - All prevading One
 Bhava - Existence, Creation
 Rudra - Dispeller of Sorrows
 Ugra - The Fearsome
 Bheema - The Tremendous
 Pashupathi  - Lord of all beings
 Mahadeva - The Supreme
 Ishana - The Directional ruler

In Linga Purana, Nandi Deva describe these eight forms:
Sarva is one Form of Shiva spread all over the Universe whose wife is stated as Vikeshi and their son is Mangala. Bhava with Uma as his wife and Shukra as their son. Agni Swarupa Shiva is called Pashupati, whose wife is known as Swaha and their son is Shanmukha. Pavana (Vayu) Swarupa Mahadeva is called Ishana whose wife is known as Shivaa and their son is Manojava or Hanuman. Rudra is the name of Shiva whose Swarupa is of Surya Deva and Suvarchala is the name of his wife and Shanaischara is their son. Soma Swarupa Bhagavan is called Maha Deva, with Rohini as his wife and Budha as their son. Yajamana Swarupa of Shiva is Ugra Deva and his wife is called Diksha and their son is Santana. Bhagavan Shiva's eighth form is known as Parthiva, signifying the hardened and cruel feature of Shiva as his wife is Prithvi and their progeny is of humanity. Tatwajnas (those who know of tatwas) believe that Pashupati is the life-provider of all Dehaadharis or Beings with Physiques; Ishana is the provider of Shobha or Charm; Bhima is that Form of Shiva who bestows Teja or radiance to various parts of body like eyes, skin etc. while Chandra Swarupa of Shiva who rules the mind of the Beings is called Rudra and Yajamana Swarupa governing Buddhi or intellect of the Beings is designated as Maha Deva.

The rich symbolism correspond to number 8 in Shaivism. Even the Shaivite philosophy has eight systems. In this case, one of main understanding that these 8 forms represent the 5 tattwas, the Sun, the Moon and the Jivatma. The Sun and Moon are the life-givers, and the Jivatma is a reflection of the Purusha-atma.
It brings to mind the story from the Madhaviya Shankara Dig Vijayam where Adi Shankara meets Lord Shiva as a chandala accompanied by the four vedas in the form of four dogs. And during the panchayatana puja, the worship of the Ashta murtis includes Adi Shankara himself. It is the delightful concept but set of others exist also.

We can see here the traditional extrapolating of the number eight as Guardians of directions (Ashta Dikpalakas).

 Sarva rules over the East
 Bhava rules over the NorthEast
 Rudra rules over the North
 Ugra rules over the NorthWest
 Bhima rules over the West
 Pashupati rules over the SouthWest
 Ishana rules over the South
 Mahadeva over the SouthEast

In the same way for Eight Bhairavas Mahakala Bhairava is the center, and His eight forms are the eight directions. Mahakala Bhairava is Shiva Himself, and the eight Bhairavas are the eight forms of Shiva in the Tantric scheme.
Also, these forms of Shiva are correlated with eight planets or assigns eight forces of nature and also the eight qualities related with these forms. Then, all world is a product of these eight forms, consists of them, and can exist and fulfill its task only because the eight embodiments cooperate.

 Sarva – for Earth
 Bhava – for Water
 Rudra – for Fire
 Ugra – for Air
 Bheema – for Space
 Pashupathi – for Soul
 Mahadeva – for Moon
 Ishana – for Sun

Because each individual is also composed of the same eight realities (e.g., the light of man's eyes corresponds to that of the Sun), Shiva constitutes the corporeal frame and the psychical organism of every living being. The eighth constituent is the indispensable performer of the rites which are devoted to gods who preside over the cosmic processes and all are Shiva's faculties.

Shaivite saint Appar has told that Shiva enjoys Puja with eight flowers "inside" and "outside" (about eight flowers - here).
Or well-known Lingaasthakam has one stanza that says:

Ashtadalo pariveshtita Lingam

Sarva samudbhava karana Lingam

Ashtadaridra vinashana Lingam

Tatpranamami Sadashiva Lingam

I bow before that Lingam, which is the eternal Shiva,

Which is surrounded by eight petals,

Which is the prime reason of all riches,

And which destroys eight types of poverty.

According to Shiva Purana and Brahma Purana these eight forms are connected to the places in India and Nepal. The following forms or forces of nature are worshipped in their primal form only without any special idols representing them.

Sarva - Bhoomi Linga, Kancheepuram, Tamil Nadu.
Bhava - Jala Linga, ThiruvanaiKoil, Tamil Nadu.
Rudra - Agni or Thejo (Divine Light) Linga, Tiruvannamalai, Tamilnadu – Arunachaleswara.
Ugra - Vayu Linga, Sri Kalahasti, Andhra Pradesh.
Bheema - Akaasha Linga, Chidambaram, Tamil Nadu.
Pasupathi - Yajamana Linga, Kathmandu, Nepal.
Mahadeva - Chandra Linga, West Bengal.
Ishana - Surya Linga, Konark Temple, Orissa.

The eight mantras of Ashtamurtis are:

OM Bhavaya Devaya Namaha

OM Sharvaya Devaya Namaha

OM Rudraya Devaya Namaha

OM Ugraya Devaya Namaha

OM Bheemaya Devaya Namaha

OM Pashupathaye Devaya Namaha

OM Eshanaya Devaya Namaha

OM Maha Devaya Samba Sachchidanandaya Namaha

In Tirumantiram, written by the Yoga Siddhar Tirumular, we read:
"The One is He, the Two His sweet Grace,
In Three He stood, in all the Four witnessed,
The Five He conquered, the Six He filled,
The Seven Worlds pervades, manifests the Eight And so remains."
 These eight forms show to us symbolically, using ancient schemes of the eight existent entities, Shiva's Divine manifestation. Thus, the Ashtamurtis together represent Shiva's universal form (Vishvarupa). He is both the Pervader and the pervaded. Only One transcendental Shiva is the center.

It has reminded me a magnificent sculpture in a CSMVS museum at Mumbai. It is a relief called Ashta Shiva or Mahadeva but we can see only 7 figures. But all murtis are united as One Linga.

OM NAMAH SHIVAYA