At the end of the play, Macbeth does something unusual: he gives one of his servants a NAME. There are a number of other messengers or gentlewomen in the play - even a doctor - but they never get names.
But then, for no reason, at the end of the play he calls out "SEYTON." Now, bearing in mind that Macbeth was meant to be seen and not read, it seems really strange to give a character a name that rhymes with Satan, in a play about witchcraft, if that character isn't going to actually be Satan.
It's also doubly interesting as he says the name three times before Seyton appears - twice in quick succession, and then suddenly he appears. The idea of saying someone's name three times before they appear is a reasonably classic trope, and here it is with Satan.
Seyton!--I am sick at heart,
When I behold--Seyton, I say!--This push
Will cheer me ever, or disseat me now.
I have lived long enough: my way of life
Is fall'n into the sear, the yellow leaf;
And that which should accompany old age,
As honour, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honour, breath,
Which the poor heart would fain deny, and dare not. Seyton!
Given the nature of the play and its focus on witchcraft - and the fact that a few scenes earlier we'd seen the arrival of Hecate, the Queen of Witches - it doesn't seem unreasonable that this is actually Satan who's arrived.
Other than ranking up the supernatural elements in the play, this doesn't really have a massive impact on the play except in one key possible area.
Look over this section from A5 S5:
A cry of women within
MACBETH
What is that noise?
SEYTON
It is the cry of women, my good lord.
Exit
MACBETH
...
Re-enter SEYTON
Wherefore was that cry?
SEYTON
The queen, my lord, is dead.
So it is Seyton who discovers the Queen's body - or, possibly, was it him or his minions who killed her?
The most common understanding is that Lady Macbeth killed herself, but bear in mind that we only assumed she did because Malcolm's says that Macbeth's "fiend-like queen, / Who, as 'tis thought, by self and violent hands / Took off her life."
So basically, it could well be that Lady Macbeth killed herself, but I'd bet there were people who left the theatre thinking that Seyton / Satan had something to do with it...