all pervading Rudra

यो रुद्रो अग्नौ यो अफ्सु य ओषधीषु यो रुद्रो विश्वा भुवनाविवेश तस्मै रुद्राय नमो अस्तु॥

कृष्ण यजुर्वेदे

yo rudro agnau yo aphsu ya oṣadhīṣu yo rudro viśvā bhuvanāviveśa tasmai rudrāya namo astu||

kṛṣṇa yajurvede

Pranams to that Rudra the Shankara

who has entered and resides in fire

who has entered and resides in waters

who has entered and resides in all medicinal herbs

who has entered and resides in the whole universe.

The mahamrutyunjaya mantra

The mahamrutyunjaya mantra is

ॐ त्र्यम्बकं यजामहे सुगन्धिं पुष्टिवर्धनं

उर्वारुकमिव बन्धनात् मृत्योर मुक्षीय मामृतात्..

om tryambakam yajaamahe sugandhim pushtivardhanam

urvaarukamiva bandhanaat mrutyormuksheeya maamrutaat

This is found in Rg Veda 7-59-12, Yajur veda Vajasaneyi Samhita 3-60 and atharvaveda 14-1-70

We worship and offer oblations to Rudra the thrayambaka( three eyed) He spreads frangrance and enhances our nourishment.

May He be pleased to realease me like the ripe cucumber from its stem, from mortal life but not from immortality

The cucumber falls down from its tendrils when it is ripe, but it has already the seeds of immortality within it.. And the separation from the mother tendril comes only when the fruit is ripe.. This would imply that by the Grace of Lord Mrutyunjaya Rudra,death may visit us with peace at ripe old age and also enable us to reach immortality at His lotus feet

In fact the mantra is found in Yajurveda Vajasaneeyi Samhita in a slightly modified form..

this is a prayer by a newly wedded lady for the benefit and longevity of her husband

ॐ त्र्यम्बकं यजामहे सुगन्धिं पतिवर्धनं

उर्वारुकमिव बन्धनात् मृत्योर मुक्षीय मामृतात् .

om tryambakam yajaamahe sugandhim pativardhanam

urvaarukamiva bandhanaat mrutyormuksheeya maamrutaat

Here the word Pathivardhanam… the enhancer of the longevity and welfare of the husband is used instead of the word Pustivardhanam..

  (yajurveda Vajasaneyi Samhita, 3-60)

may the entire universe be pleased...

ये देवासो दिव्येकादश स्थ पृथिव्यामध्येकादश स्थ | 

अप्सुक्षितो महिनैकादश स्थ ते देवासो यज्ञमिमं जुषध्वम् ||

ഏതൊരു ദേവന്മാര്‍ പതിനൊന്നു രൂപങ്ങളില്‍ ആകാശത്തിന്റെ മധ്യത്തിലും, ഭൂമിയുടെ മധ്യത്തിലും ജലത്തിന്റെ നടുവിലും നിലകൊള്ള്ന്നുവോ ആ ദേവന്മാര്‍ എല്ലാം ഈ യജ്ഞം ഭുജിക്കട്ടെ

ये देवताये दिव्यलोक, पृथिवी और अंतरिक्ष में ग्यारह के संख्या में रहते है | आप हमारी आहुति ग्रहण करे ||

this Rigvedic Mantra is chanted when food is offered to a Leanred Brahmin or a great Yati.

Here, the assumption that through their mouths, the Food in the shape of Yajna goes to all devas who are occupying the heaven, earths and the water in eleven forms..( The eleven Rudras?

And that all forces of nature are satisfied and become benevolent to the occupants of the earth.

The difference between Vishnu and Shiva 

is only our invention.. it is not envisaged in the Religion) Rigveda 1.139.11

NamaH Sivaaya

NamaH Sivaaya

मानो महान्तमुत मानो अर्भकं
मा न उक्षन्तमुतमान उक्षितं।
मा नो वधीः पितरं मोत मातरं
प्रिया मा नस्तनुवो रुद्र रीरिषः॥५२-१॥
मा नस्तोकेतनये मा न आयुषि
मा नो गोषु मा नो अश्वेषु रीरिषः।
वीरान्मानो रुद्र भामितो वधीर्
हविष्मन्तो नमसा विधेम ते॥ ५३-१॥
महानारायणोपनिषद्॥
māno mahāntamuta māno arbhakaṁ
mā na ukṣantamutamāna ukṣitaṁ|
mā no vadhīḥ pitaraṁ mota mātaraṁ
priyā mā nastanuvo rudra rīriṣaḥ||52-1||
mā nastoketanaye mā na āyuṣi
mā no goṣu mā no aśveṣu rīriṣaḥ|
vīrānmāno rudra bhāmito vadhīr
haviṣmanto namasā vidhema te|| 53-1||
mahānārāyaṇopaniṣad||
māno mahāntamuta māno arbhakaṁ
mā na ukṣantamutamāna ukṣitaṁ|
mā no vadhīḥ pitaraṁ mota mātaraṁ
priyā mā nastanuvo rudra rīriṣaḥ||52-1||
mā nastoketanaye mā na āyuṣi
mā no goṣu mā no aśveṣu rīriṣaḥ|
vīrānmāno rudra bhāmito vadhīr
haviṣmanto namasā vidhema te|| 53-1||
mahānārayaṇopaniṣad||

This is a prayer to Sri  Rudra, the Lord Shiva the Destructor among the  triumvirate ..this is found in Mahanarayanaopnishat,  Taiittareeya aranyaka of Krishna yajurveda. It is also chanted as part of Sri Rudram 

the free translation is
Oh, bhagavan Rudra..
 please do not injure or punish  the elders, nor the infants, 
Do not hurt the adults  who are endowed with the capacity to procreate 
nor the foetus lying in the female wombs. 

Do not injure our dear fathers or mothers. 
 Do not destroy our  bodies.
Oh Rudra we offer oblations to you through havis (cooked grain) 
and we offer you utmost reverence.. 
Be pleased not to hurt us through harm to our children, 
to our grandchildren.
 Be pleased not to hurt other persons belonging to us, 
our cows and cattle, and our horses. 
 Be pleased not to annihilate our heroes..who protect us from attacks.

In the nomadic life in the Vedic period man feared the natural forces and the destructive hands of the Rudra terrified them.  
So they offered praise and oblation to the angry God with a prayer that  all that is near and dear to them should be left unharmed. 
The Sreerudraprashnam in yajurvedam  also called Shatarudreeyam and Namkam is also  a prayer to Lord Shiva who has manifested in the fierce form of Rudra..

and the Chamakam following it is the recital of a long list addressed to the same lord,  of things we woud be bestowed with , 
when the  Lord has abandoned  his anger
and is in a pleasant and blessing mood.  

It is no wonder that Lord Shiva is called Aashuthosha...who become calm and happy very easily.  
The Lord's anger is towards evil and when his children cry in supplication before the primordial father, like all nice fathers, his anger subsides in a jiffy and he is prepared to do anything for his children

NamaH Sivaaya