At the very top Overlord Redgit resides. He requests that a great evil dimension(The Shallows), and their leader, the Onetheci, be banished from existance. He also has other quests for you to do. You are not awarded with the rewards unless you return to him. If you complete all quests, including slaying the Onetheci, he will become a Town NPC, but can only live in room/house made out of Lightstone Brick. If you complete a quest without knowing it, the you talk to him, you must do the requirements again after you met him.
-Getting Started: Kill 5 slimes. Reward: 3 Gold Bars.
-Extractor: Mine 25 Copper, Silver, Iron, and Gold Ore. Reward: Regeneration for 30 minutes.
-Underworld Retriver: Kill five Voodoo Demons, and give him the dolls acquired from the demons. Reward: 3 Helping Hand Potions.
-Seeker Beyond What The Eye Can See: Enter the Shallows, and come back via death. Reward: 10 Platinum Coins.
-Destroyer: Kill 2000 Enimies of any type. Reward: 50 Gold Coins.
-Duengon Run: Go into the Duengon for 10 minutes without a weapon, and come out alive. Reward: 15 Greater Healing Potions, 8 Platinum Coins.
-Finish The Darkness: Kill the Onetheci. Reward: Heaven's Might, 50 Platinum Coins.
Due to request by someone in the comments, this is now a section.
Holy Guards: Holy Guards are found on every floor, attack one, the whole army chases after you.
Light Paladin: A stronger version of the Holy Guard, strike one, and death is almost sure.
Juggernaut of Peace: Pretty much complete opposites here. You cannot say Juggernaut and Peace in the same sentence. The strongest there is.
-Redgit is actually the creator of Terraria, referencing him kind of as a god of the Terrarian world.