Cthulhu used to be a regular splicer until he lured by the Legion with promises of power. They convinced him to do genetic experiments on himself fusing Legion D.N.A and the Blue Pandemic virus with his D.N.A giving himself a monsterous form similar to his namesake. Cthulu broke free of the Legion's control and took control of them The Legion followed him because they believied he would lead them to take over Rapture and the ADAM they desired. He used his new army to attempt to overthrow the Japanese Governer Ryu Takeda. However unlike most of the revolutionary factions which fought for political reasons Cthulhu desired only the limitless power Rapture's ADAM supply offered. He almost succeded in taking power and defeating any opposing factions however the Blue Pandemic virus in his system eventually got to him and he died before acheiving his ultimate goal of taking over the city. The Legion was deafeted and their population was reduced by 80%. What was left of The Legion retreated back to the old areas of Rapture after their master died from his sickness during his planned endgame were they waited for another chance to gain power.