A Rohan is a NPC(non-playing character) that sells the best items any NPC does when he comes into your world you automatically get a Rohan voodoo doll when you equip this you can hit the Rohan when you kill him you will get ths message the following night 'You will regret what you did later on the message will apear Cthulhu prime has awoken at this moment you will have a 0.00000000000000001%  living that night Cthulu prime is a powerful boss which has been rated the strongest boss the second being the Skeltron prime.You will probably want to keep the Rohan anyway because he sells you amazing stuff such as : demonite pickaxe,suspicious looking eye,crimtane/demonite bars and demon bow. to get a Rohan in your world you need to be in hard mode.