A very simple and yet beautiful and powerful Mantra of lord Shiva is the Rudra Dwadashanama Stotra.
Rudra dwAdasha NAma StOtra
Prathamam tu MahAdEvam DvitIyam tu MahESvaram|
tritIyam Sankaram prOktam chaturtham Vrishabhadhwajam||
Shiva is known by twelve names. Mahadeva is the name which speaks of him as the great lord. Maheshwara signifies that he has all auspicious divine powers in him. Shankara signifies that he always does good to his devotees. He is Vrishbhadhwaja since he has Nandi, the bull flag as his insignia.
Panchamam KruttivAsam cha ShaShTham KAmAnganASanam
saptamam dEvadEvEsham SrikanTham chAShTamam tathA
Shiva adorns with tiger’s skin and hence is Kruttivasa. He burnt Manmatha to ashes and hence is KAmanganAshanam. He is the overlord of other gods and hence is DEvadEvEsha. He consumed the poison which originated from the ocean, and hence is known as Srikantha.
Navamam tu haram dEvam daSamam pArvatIpatim |
RudramEkAdaSam prOktam dwAdaSam Sivamuchyate||
He cleanses all sins and hence is Hara. He is the Parvati’s husband and hence is Parvatipati. He made a cry during creation and hence is Rudra. He is most auspicious and hence is Shiva.
EtaddwAdaSanAmani trisandhyam yah paThetnarah|
gOghnaSchaiva krutaghnaScha bhrUnah gurutalpagaH||
strIbAlaghAtakaSchaiva surapo vruShalipatiH|
Sarvam nASayate pApam Sivalokam sa gacchati||
One who chants this 12 names of Shiva during three twilight hours shall be bereft of sin of killing a cow, being a cheater, killing an unborn child, cheating a teacher, killing a women, killing a child, consuming liquor, marrying an unsolicited woman.
SuddhasphatikasankASam trinetram chandraSEkharaM|
IndumanDalamadhyastham vande devam sadASivam||
Lord Shiva shines with the lustre of a clear crystal and has the crescent of the moon on the head. He is also present in the moon’s orb and I salute the great god Shiva who is ever auspicious.