In a play that is essentially about Prospero getting his revenge on those who exiled him so long ago, it is interesting that it is finally Ariel that has to draw his lord and master's attention to the way in which he has succeeded in his goal and has awakened contrition in some of his prisoners. At the beginning of Act V scene 1, Ariel reports to Prospero about how his "project" is going and whether he is succeeding in his goal or not. It is Ariel in this scene that seems to move Prospero towards showing mercy. Note how he reminds Prospero that the hour has arrived when Prospero said "our work should cease," and paints a pitiful picture of the King and his followers:

The King,

His brother, and yours abide all three distracted,
And the remiander mourning over them,
Brimful of sorrow and dismay; but chiefly
Him that you termed, sir, the good old Lord Gonzalo.
His tears run down his beard like winter's drops
From eaves of reeds. Your charm so strongly works 'em,
That if you now beheld them, your affections
Would become tender.

Note the simile used by Ariel to evoke our pity and Prospero's for Gonzalo, whose tears, we are told, run down his beard "like winter's drops / From eaves of reeds." At every stage, Ariel shows the success of Prospero's plan and makes sure that Prospero is given all the glory, whilst trying to evoke a feeling of pity and sympathy in his master. Ariel supports this by saying that if he were human, he could not help but feel sorry for them.

It is based on this report from Ariel that Prospero decides to show mercy, saying "The rarer action is / In virtue than in vengeance." Now that these men have shown themselves to be "penitent," he resolves to free them and reveal himself.