[The Might]

Form, I checked the letters, but couldn't find the reference you mentioned.

Looking at the Silmarillion it is interesting to note that "thee" and "thine" are mostly used by Eru and by the Valar. It makes sense since the Valar and Eru were close to one another.

Eru to Aule:

But when the time comes I will awaken them, and they shall be to thee as children; and often strife shall arise between thine and mine, the children of my adoption and the children of my choice.

Aule to Eru:

Yet the making of thing is in my heart from my own making by thee; and the child of little understanding that makes a play of the deeds of his father may do so without thought of mockery, but because he is the son of his father. But what shall I do now, so that thou be not angry with me for ever? As a child to his father, I offer to thee these things, the work of the hands which thou hast made.

Mandos to Feanor:

But after that time this matter shall be set in peace and held redressed, if others will release thee.

Melkor to Ungoliant (perhaps also emotional? )

Therefore Melkor said to her: 'Do as I bid; and if thou hunger still when all is done, then I will give thee whatsoever thy lust may demand. Yea, with both hands.'

Later:

'Thou hast had thy doe. For with my power that I put into thee thy work was accomplished. I need thee no more.

A messenger of Manwe to Feanor:

Then thou hast sworn in vain, for none of the Valar canst thou overcome now or ever within the halls of Eä, not though Eru whom thou namest had made thee thrice greater than thou art.'

Ulmo to Turgon:

And Ulmo warned Turgon that he also lay under the Doom of Mandos, which Ulmo had no power to remove. 'Thus it may come to pass,' he said, 'that the curse of the Noldor shall find thee too ere the end, and treason awake within thy walls. Then they shall be in peril of fire. But if this peril draweth nigh indeed, then even from Nevrast one shall come to warn thee, and from him beyond ruin and fire hope shall be born for Elves and Men.

Manwe to Melkor:

'This kingdom thou shalt not take for thine own, wrongfully, for many others have laboured here do less than thou.'

The other two passages with extensive use are Gwindor to Turin as a friend and Glaurung to Turin in the way Denethor and Mouth of Sauron use it.


PS: Sorry, I just noticed the thread title says "in Lord of the Rings", I hope adding the Silm to the dicussion is ok.