Being a Black Templar, my seals has a lot of prayer-type oaths on them. Mine essentially are speeches given before battle, made into seals to remind the marine of them. I went with the four Templar vows plus 4 homemade ones.
Lead us from death to victory, from falsehood to truth.
Lead us from despair to hope, from faith to slaughter.
Lead us to His strength and an eternity of war.
Let His wrath fill our hearts.
Death, war, and blood;
In vengeance serve the Emperor and the name of Dorn!
Trust in the Emperor at the hour of battle.
Trust to him to intercede, and protect his warriors true as they deal death on alien soil.
Turn their seas to red with the blood of their slain.
Crush their hopes, their dreams
and turn their songs into cries of lamentation.
Smite now the scions of the Witch!
Grant us the strength to pierce their unclean flesh!
To cover their fields with the pale form of the blasphemous dead!
To drown the thunder of guns with the shriek of their dying!
To lay waste to their citadels with hurricanes of fire!
To wring the hearts of their kin with unavailing grief!
To send them into the waste of their desolate land in rags and hunger, broken in spirit, worn with travail and begging for the refuge of the grave.
We ask it, in the spirit of wrath, O Master of Mankind!
O Emperor, in wrath rejoicing at bloody wars, fierce and untamed,
whose mighty power doth make the strongest walls from their foundations shake.
All-conquering Master of Mankind, be pleased with this war