In my post on this subject on 9 May 2020 I did not comment on Sam's use of 'stern' to describe how he saw Frodo 'with other vision' both in the Emyn Muil and on Mt Doom. The persistence of this single word unaltered, while the rest of the description changes and darkens, makes it stand out quite proudly when comparing the two visions. Here are the relevant phrases once more:
a tall stern shadow, a mighty lord who hid his brightness in grey cloud
stern, untouchable now by pity, a figure robed in white, but at its breast it held a wheel of fire.
Sternness is of course commonly associated with a kingly or authoritative manner or appearance. Aragorn, Gandalf, Boromir, Faramir, Denethor, Théoden, and Éowyn are all called stern. Frodo is repeatedly called stern as he tries to tame Gollum by means of the Ring (TT 4.i.616, 618 twice;.iii.640, 643). Yet the word has another sense, one long obsolete and unfamiliar to most people today. In Middle and Early Modern English, in authors from Layamon to Shakespeare, it could also mean merciless or cruel. We need look no farther than Tolkien’s own edition of Ancrene Wisse for two most pertinent examples of precisely this sense of the word:
‘He dude him seoluen bitweonen us & his feader þe þreatte us forte smiten ase moder þe is reowðful deð hire bitweonen hire child ant te wraðe sturne feader hwen he hit wule beaten.’
‘He put himself between us and his Father who was threatening to smite us, just as a mother who is merciful puts herself between her child and the wrath of a stern father when he wishes to beat him’.
In this first passage we see the sturne feader contrasted with the moder þe is reowðful. Given the father's intent and the mother's compassion or mercy, 'cruel' would be accurately render sturne here.
Rihtwisnesse, he seið, mot beo nede sturne, ant þus he liteð cruelte wið heow of rihtwisnesse. Me mei beon al to riht wis. Noli esse iustus nimis. In ecclesiaste.‘Justice’, he says, ‘must necessarily be stern’, and thus he dyes cruelty with the hue of righteousness. But one may be all too righteous. ‘Be not excessively just.’ [It says] in Ecclesiastes.
That ‘untouchable now by pity’ follows 'stern' immediately further argues that by the time they reach Mt Doom Frodo’s sternness has gone beyond righteousness towards cruelty. For given the cardinal importance Gandalf assigned to pity and mercy in The Shadow of the Past, a lesson Frodo refused to learn until his own sufferings and the wretchedness of Gollum taught him otherwise, to be beyond pity’s touch simply cannot be good.
Moreover, there is less to Frodo’s appearing to be ‘a figure robed in white’ than meets the eye. In the Emyn Muil Sam saw him as ‘a mighty lord’ who shone with a light from within, something Sam and Gandalf have seen Frodo do before. Here on Mt. Doom the most important element in the vision is the Wheel of Fire. Here it is the Ring which blazes with a light of its own and which has transfigured the ‘mighty lord’ into an indistinct ‘figure’, an ‘it’ rather than a person.
The whiteness which robes the figure is also ambiguous, since whiteness can be deceiving. When Frodo becomes able to see beneath the cloaks of the Black Riders, he sees them and their clothing as grey and white (FR 1.xi.195; xii.213). Saruman the White also appears white, when he is so no longer. The dismissive 'but' that follows at once identifies the Wheel of Fire as by far the more important detail. In the end the Wheel is what remains, and thus it is from the Wheel that the voice seems to speak, loud and cruel.