To get this Achievement you have to kill Satan on the Floor "Sheol" which is found after the Womb/Utero
In order to unlock Satan you have to defeat the depths at least once.
Sheol only has a chance of spawning whenever you defeat Moms Heart. However, after you defeat "It Lives" Sheol has a 100% chance to spawn.
The Satan Fight has 3 phases to it.
1. - First the boss "The Fallen" will spawn in along with 2 Kamikaze leeches. After "The Fallen" is defeated then Satan will spawn in phase 2
2. - Satan will have 4 set patterns that you will have to get used to in order to defeat him. 1.) Projectictiles that move to the left and right of the room. 2.) brimstone laser down the middle. 3.) Homing Brimstones from his hands. 4.) Scattering shots simular to Gurdy's attack from his mouth directed at isaac.
3. - Satan will begin to stomp around similar to Mom's Foot, but much quicker. The second foot will come soon after the first one. 2 Kamikaze leeches will spawn as well and will continuously spawn another after one of the leeches is defeated.
After this phase is defeated you will have defeated Satan and unlock Judas
Credit goes to: Insane Fallout on www.trueachievements.com