In the far, far northern end of the Capital Wasteland, almost directly in the middle of the map (and due far, far north of Megaton) is an isolated and hard-to-locate settlement called Oasis. Lawn Mower Blade, Tin Can (M), Empty Soda Bottle (M), Grimy Pre-War Businesswear, Crutch (x2), Darts (M), Pre-War Book, Bottle Caps (M), Clipboard, Nuka-Cola, Plunger, 10mm Rounds (M), Silenced 10mm Pistol, Ashtray, Abraxo Cleaner, Flamer Fuel (M), Missile (x2), Mini Nuke. (3) (Optional) Kill Harold by burning him. (2) Enter the caves below and destroy Harold's Heart. If you do what Harold wants you to do, and kill him by destroying his heart, you will get a new Unique Perk, Barkskin.(1) Undertake the Treeminder's Ceremony.
Harold with also emit a terrible and unearthly shriek. If you do this you will gain negative Karma, and everyone in the Oasis will become hostile to you. Another option to kill him is to light him on fire using a fire-based weapon. Once you find his heart, you can destroy it, like Harold wants you too, or if you have talked to others in the Oasis, you will have the option to slow down Harold's Growth, or Accelerate it. He tells him that he thinks Herbert brought his organs down to the roots, so you have to venture down into a tunnel filled with Mirelurks and Mirelurk Kings. He asks the Lone Wanderer to kill him because he is just tired of life. In Fallout 3 he ventures all the way to the Oasis and soon, apparently, Herbert/Bob got too heavy, so he plopped down, and soon Herbert/Bob grew bigger until he was how we all know and love 'em, Harold: The Talking Tree. I talk to him sometimes when I get lonely." Then he will say to you after you make a comment, "Ah! Just kidding! His name's Bob!" If your character asks "Is that a tree growing out of your head?" he will answer, "You like him? His name is Herbert. He now has a very small tree growing out of his head.
In Fallout 2 you find him being the leader of a town of Ghouls called Gecko. Harold in Fallout 2, with the tree first starting to grow. (Forced Evolutionary Virus) and became the Master, but Harold doesn't tell you this because he is unaware. He tells you the story of how he became what he is and how Richard was hit by a crane and Harold never saw him again. He looks like a ghoul and seems to have grass growing out of his head. In Fallout 1 you find him as an old hermit in The Hub.