Danzo Shimura was one of the Leaf Village's elders, leader of the ANBU Black Ops sub-division known as "The Foundation" and was appointed as the Candidate 6th Hokage after Pain's Invasion.
Danzo had a deep seated hatred towards the 3rd Hokage, Hiruzen Sarutobi, and began planning ways to someday attain the title himself. To that end, he created the ANBU sub-division named "The Foundation". During the 3rd Great Ninja War, Danzo agreed to help Hanzo of th Hidden Rain Village in order to receive help in claiming the title of Hokage. He sent some of his forces to deal with the Rain Orphans but they were all wiped out by Nagato.
At some point after Shisui Uchiha was killed, Danzo implanted Shisui's right Sharingan and arm into his own body. He also worked with Orochimaru to implant Hashirama Senju's DNA into his right arm, modifying his physical energy. Madara speculated this was done to allow Danzo to use his eleven Sharingan at once, with the additional effect of allowing Danzo to use Wood style ninjutsu and ultimately the forbidden Dojutsu, Izanagi.