Gargauth is a pit fiend trapped in the Shield of the Hidden Lord. It is a seed of corruption in the mortal world but Avernus is so corrupt it hardly registers. Gargauth can only be freed by a suitable powerful attack against the shield. Tiamat's breath, the Sword of Zariel, or the fires of Bel's Forge could break the shield. Gargauth knows where the Bleeding Citadel is.