Ah, the underworld. An area with the last stuff available in easymode. Ill be telling you about the hazards and loot in this area.
The Underworld is at the bottom of the map, under the stone layer. You can dig straight down, creating a "Hellevator" but you may run into lava and water which would make it much harder to dig. I used the underground jungle as a passage, as it often stretches to the Underworld. If you are particularly ambitious, you can blast your way there with bombs and TNT purchased from the demolitionist.
It's usually suicide to go there without shadow armor, as it will protect you from the many hazards there. A water walking potion or obsidian skin is useful for navigating the lava seas. An ivy whip for navigation has its merits.
For accessories, you need an obsidian skull (For Hellstone) and your choice of other accessories. I went there with the skull, a cobalt shield (getting knocked into lava is not fun) rocket boots (mobility) feral claws (for dps) and an agley of the wind (the warding prefix was awesome)
For weapons, a muramasa or blade of grass for melee. For ranged i recommend a handgun with normal musket balls. If you are a spell caster, a combo of water bolt and magic missile is great.
The mobs, lava and Hellstone could prove to be deadly. Not paying attention could cause Hellstone or lava to leak onto you, possibly knocking you into more enemies or lava. There are no traps, though lava could be considered one.
Just like other slimes, no real threat here except possibly being knocked into lava.
One of the most annoying enemies in the game. While you are mining ores, they pester you with mid-damage fireballs. Luckily, you can kill their fireballs with a pickaxe. You can probably just go up to them and kill them, as they don't have that much health.
A large worm type enemy, they aren't too strong, but can be annoying. Anyway, a spear or good pierce weapon can kill it easy.
Just a normal old annoying bat. No problem.
Demon & Voodoo Demon
Nothing too outstanding, shooting demon sickles, and having a bit of health. They are kind of strong, but should be no problem if you're properly geared.
The flame lash and flower of fire are here, they are powerful well into HARDMODE, so grabbing shadow keys from the dungeon is a good idea.
Just the dark lance, which is worth grabbing due to its help against wraiths in the opening minutes of hardmode.
Hellstone is the only ore here, be carefull while mining it because it becomes lava. Cook it in a hellforge and you'll have access to the molten set, you will need the pickaxe to mine cobalt. This ore makes some of the best stuff in easymode, so grabbing it before the WoF is a good idea.
I will cover the WoF in my hardmode guide.