namaste bhagavan rudra bhāskarāmitatejase.
namo bhavāya devāya rasāyāmbymayāya te..
śarvāya kṣitirūpāya sadā surabhiṇe namaḥ.
iśāya vāyave tubhyaṃ saṃsparśāya namo namaḥ.
paśunāṃ pateye caiva pāvakāyātitejase..
bhīmāya vyomarūpāya śabdāmātrāya te namaḥ.
mahādevāya somāya amṛtāya namo’stute.
ugrāya yajamānāya namaste karmayogine..
yaḥ paṭhed śṛṇuyadvāpi paitā mahamimaṃ stavaṃ.
rudrāya kathitaṃ viprāñśraāvayedvā samāhitaḥ..
aṣṭamūrtestu sāyujyaṃ varṣādekādavāpnuyāt.
The first attestation of this aspect(rather,collection of aspects of Śiva) is in the Śāṅkhāyana Brāhmaṇa,where Rudra,on his own request after being generated by Prajapati,takes on 8 names: Bhava,Śarva,Paśupati,Ugra,Mahādeva,Rudra,Iśāna and Aśani,respectively the waters(āpaḥ),Agni,Vāyu,plants and trees,the sun,the moon,food and Indra respectively. The text then ends with “sa eṣo’ṣṭanāmāṣṭadhā vihito mahān devaḥ“. Also,a similar attestation exists in the Śaṭpāṭha Brāhmaṇa where he takes teh epithets Rudra,Sarva,Paśupati,Ugra,Aśani,Bhava,Mahādeva and Īśāna each manifested through a rūpa. In this version,Agni becomes the form of Rudra,the waters(āpaḥ) of Śarva,the plants of Paśupati,Vāyu of Ugra,lightning of Aśani and the sun of Īśāna.
The Liṅga Purāṇa version(mentioned here) identifies Rudra with the sun,Bhava with the waters and taste,Śarva with earth and smell,Īśa with vāyu and touch,Paśupati with fire(pāvaka),Bhīma with ether,Mahādeva with candra(the moon) and Ugra with the yajamāna.
It is also attested in the Anuśāsana parvan of the Mahābhārata(ch.16)
bhūr-ādyān sarvanhuvanān-utpādya sadivaukasaḥ.
dadhāti devastanubhuraṣṭābhiryo bibharti ca..
In South India,it is first attested int he Maṇimekhalai epic where it talks about the Śaivavādin,the expounder of the doctrine of eight forms of Īśvara.
Manikkavāsagar also attests this in his Tiruvāsagam,Appar and Sambandhar.
Kālidāsa in his benedictory verse of the Abhijñānaśakuntalam mentions this concept too
yā sṛṣṭiḥ sraṣṭur-ādyā vahati vidhihutaṃ yā havir-yā ca hotṛ
ye dve kālaṃ vidhattaḥ śrṇotiviṣayaguṇā yā sthitā vyāpyā viśvam।
yām-āhuḥ sarvabījāprakṛtir-iti yayā prāṇinaḥ prāṇavantaḥ
pratyakṣābhiḥ prapannas-tanubhir-avatu vas-tābhir-aṣṭābhir-Īśaḥ॥
This aspect of Shiva is also found in the famous Shiva Mahimna Stava.
bhavaḥ śarvo rudraḥ paśupatirathograḥ sahamahāṃstathā bhīmeśānāviti yadabhidhānāṣṭakamidam।
amuṣminpratyekaṃ pravicarati deva śrutirapi priyāyāsmai dhāmne pravihitanamasyo’smi bhavate॥
In Indian epigraphy,this concept finds mention in the following places
* Kurgod inscription of the Cālukya prince Someśvara IV and the prince Rācamalla II(dated 1173 and 1181 CE respectively).
* Devapattana Prashasti of Śridhara()
* Ratta inscription from Saundatti(dated 1229 CE)
In Indian iconography,the Mūrtyāṣṭaka is mentioned,but their depiction is surprisingly uncommon. However,they are invoked and beheld in the bricks when a Śiva temple is built.
Now,as to the epigraphical evidence in East Asia
* Phnom Preah Vihear Stele of Bhavavarman II,Śiva is invoked thus
jayāt īnduravi vyomavāyvātmakṣmā-jalānalaiḥ
* Tuol Ang Tnot inscription of Jayavarman I(681 CE)
* Bakong Stele inscription of Indravarman I(881 CE)
* Mebon and Pre Rup steles of Rajendravarman(952 and 961 CE): Installations of Aṣṭamūrtis are alluded to. Symbolized by 8 liṅgas in 8 sanctuaries grouped together.
The relevant texts of the Mebon stele
yenaitāni jaganti yajva-hitabug-bhāsvan-nabhasvan-nabhaḥ-
* Prasat Khna inscription of Udayadityavarman II(1060 CE?)
(Note:The concept of the identity of the ātman and the yajamāna is also implied here.)
* Sdok Kak Thom Inscription of Udayadityavarman II(1052 AD). The Śivācārya would offer a garland of 8 flowers(yo-dāt svayaṃ pratyahaṃ aṣṭapuṣpin-tanūnapāto-ṣṭatanoś ca tuṣṭyai)
* Banteay Srei Inscription of Jayavarman V(968 CE) refers to an offering of an 8-flowered garland by the rājaguru Yajñavarāha.
* My-Son Stele inscription of Prakāśadharma-Vikrāntavarman I(dated 657 CE)
* Another inscription by the same king Vikrāntavarman also at My Son:Here,Siva is described as the deva “whose image,identical with the Universe,is manifested by His forms—earth,water,fire,air,sky,sun,moon and sacrificer,which are fifnified by the titles of deva of great power,named Śarva,Bhava,Paśupati,Īsāna,Bhīma,Rudra,Mahādeva,and Ugra(avanivana-panakasakha-pavanavanadapatha-daśaśatakiraṇa-śatakiraṇa-dīkṣita-tanubhir—atanuprabhāvābhiḥ śarva-bhava-paśupat-īśāna-bhīma-rudra-mahādevogrābhidhāna-pradhāna-samupabṛṅhitābhir—āvirbhāvita-viśvamūrtinā)
* Canggal inscription of Java(describes Rudra as sustaining the world through His eight bodies)
* An aṣṭamukhaliṅga exists at Bali(was dated sometime between the 10th-14th century) at Pedjeng.