East Oblivion Gate:
Started: Botternil , Farm District
Items Needed: None
Items Recomended: Weapons, armor, food
Reward: Sigil stone, diamond swords, diamond armor, diamond shovel, diamonds, gold ingots, random blocks, golden apples, bread and torches.
Go to Botternil , at the Farm District you will find the knight Mariah at some kind of outpost looking for some help, she will talk you about all the deaths of her legion and will give you some supplies to you can close the Oblivion Gate attacking Botternil .
Go to the Oblivion Gate , should be really easy to see from the Farm District, watch around, they are Ghasts and zombie pigmen spawners, get inside the nether realm and find your respective nether tower (the east oblivion gate tower), at the first floor you will find zombie pigmen, make your way to the top.
Watch out, some floor may be destroyed and without light, so look down.
Reach the top, DON'T try to kill the boss, you will waste your time, just take the sigil stone (slime ball) and get out of the tower.
For more quests, visit the Quests article.