The lost soul is a special character generation option in Slash'EM Extended. To play as one, the player needs to turn on the "lostsoul" option before the game starts, which is done via the OPTIONS=lostsoul line in the configuration file. This puts the character on Medusa's level instead of dungeon level 1, adds some identify scrolls and food rations to their inventory, and prevents any means of levelport or branchport. The player's goal is the same as usual: get the Amulet of Yendor and ascend; however, starting out at dungeon level 36-39 (depends on the exact depth of Medusa) as a level 1 character means most of these characters will die quickly. The standard strategy would be to try to get up to shallow dungeon levels quickly, relying on a lot of Elbereth and hoping for good stair placements; most random monsters will be out of depth for this character and cause a quick YASD.
This idea has been derived from ToME (Tales/Troubles of Middle Earth), where the character generation dialog offers the choice of playing a Lost Soul. In that game, the player would start in the Halls of Mandos, a very difficult dungeon level, as a level 1 character that dies if something sneezes at it, often generating the "It breathes -More- You die..." message.