Today's comic is mostly an obscure conglomeration of in-joke, because sometimes you have to do that.
The passage in question is this one, from Macbeth:
And Duncan's horses--a thing most strange and certain--
Beauteous and swift, the minions of their race,
Turn'd wild in nature, broke their stalls, flung out,
Contending 'gainst obedience, as they would make
War with mankind.
'Tis said they eat each other.
They did so, to the amazement of mine eyes
That look'd upon't.