After defeating various Dark Justiciars and Umbral Tremors outside Balthazar's room in the Gauntlet of Shar, the player can enter and talk to Balthazar. During the course of conversation, Balthazar can be asked about Flesh, and will reveal that it is partly made up of pieces of his unborn twin brother.
The player can gain the ability to summon Flesh as a temporary ally if they ask Balthazar for supplies and succeed a DC 14 Persuasioncheck. He will then give the player the Summon Golem Bell which allows them to summon Flesh as an ally in battle anywhere inside the Gauntlet of Shar.