Snape, Dumbledore, and Voldemort are copyright J.K. Rowling.
'Invictus' is a poem by William Earnest Henley. I am aware that it was quoted by Timothy McVeigh before his execution, but that's hardly the poem's fault, now is it?
Invictus. It means 'unconquerable'. It is also the title of a rather pompous and overblown poem (in Snape's opinion), which Dumbledore has had framed and given to him.
Under the bludgeonings of chance, my head is bloody but unbowed. His head has been bloodied often. It has been bowed, as well, to kiss the Darklord's feet.
Snape dislikes self-pity. Still, he reads the poem and thinks of the vicious monster he once worshipped, and the white wizard he now serves, and cannot help but laugh, bitterly.
I am the Master of my Fate. I am the Captain of my Soul.