User blog comment:NickTheStick248/Talkin & Walkin/@comment-3301714-20120519095824

I reccomend you at least have a Phaseblade or a Blade of Grass and at least Shadow Armor before you try to venture into the Underworld, or you will die quite easily from the Demons and such. Anyway, you ought to bring back Hellstone, which is a bright red ore-ish looking block. If you are looking to mine it, you should have an Obsidian Skull as well, or you will take damage from the hellstone. Also, watch out for the lava that spawns when the hellstone is mined. If you are looking to build stuff with materials from the underworld, you should get Ash Blocks, the main block in the entire underworld, Obsidian Bricks and Walls , depending on what you plan to build. In case you don't know, you need 1 Obsidian and 4 Hellstone to make Hellstone Bars, one of the most valuable items Pre-Hardmode. Note that you also need a Hellforge to smelt the items into bars. Hellforges can be found inside Underworld Houses and collected with a Meteor Hamaxe or higher. Hope this helps!