This mod adds a block called reinforced obsidian, that can't be broken by the Wither. Thus, it allows the player to build a safe Wither cage with as little as 6 blocks. It also adds dark iron blocks, ingots and bars used to create the reinforced obsidian
The block itself comes from EnderIO and although the textures are similar, I recreated them in paint.net myself. This also means that they are subject to change.