Ragnarok, “The Doom of the Gods”, is the name the pre-Christian Norse gave to the end of their mythical cycle, during which the cosmos is destroyed and is subsequently re-created. “Ragnarok” is something of a play on words; an alternate form, which sounds almost identical when spoken, is Ragnarøkkr, “The Twilight of the Gods”.
To preprare for this event Odin selected the ablest human warriors to join him in the final battle against the world-devouring giants. The image on the T-shirt portrays Odin fighting the giant sea serpent Midgardsormr, who ultimately causes a deluge resulting in submersion of the world. Afterward, the world will resurface anew.