Misericordias Domini in aeternum cantabo is the beginning of the psalmus of the Introit (Officium in Sarum terminology) of the Votive Mass of the Five Wounds of Jesus. This was one of the most popular Votives used in Medieval England ("drill into it", as the inimitable Fr Zed would say, by looking for it in the index of Duffy's Stripping of the Altars). Here is a translation of the introduction to it in the Sarum Missal:
"S Boniface the Pope was sick even unto death; and he urgently begged of God that his life in this world be prolonged. The Lord sent to him S Raphael the Archangel with the Office of the Mass of the Five Wounds of Christ, saying to the Pope:
'Get up and write this Office; and say it five times; and immediately you will receive your health. And whatever priest shall celebrate this Office five times for himself or another sick person, he shall receive health and grace, and in the future he will possess eternal life, if he perseveres in good. And in whatsoever tribulation a man shall be in this life, if he procures of a priest this Office to be read five times for himself, without doubt he will be set free. And if it is read for the soul of a Departed, immediately after it shall have been completely said, that is to say, five times, his soul will be loosed from pains ...
'Then Pope S Boniface confirmed the Office by Apostolic Authority, granting to all truly confessed and contrite, the seventh part of the remission of all their sins if they should have read it devoutly five times ..."
It was an enormously popular Mass among both clergy and laity (particularly when the latter were making wills). It is not surprising that Master Patrick Haliburton [he was a MA of S Andrews] had these familiar words carved on his choir stall: I will sing for ever the mercies of the Lord.
But - call me an typical Enlightenment sneering sceptic if you must - I don't entirely believe the story about S Boniface and the Archangel. I'll tell you why soon.