Tongue of Acid is an enhanced long sword especially useful in the final battle of 
Baldur's Gate:
Siege of Dragonspear (2016)
This icon indicates content from the Siege of Dragonspear campaign of the Baldur's Gate: Enhanced Edition.. It is obtained as one of the three rewards by answering correctly ("murder") in Thrix's riddles, other two choices are Echo of the Fiend, or The Guardian Devil.
Description[]
| “ | This infernal longsword, forged to resemble a devil's tongue, has a blade "sharper than any scream" according to the writing on the hilt. Its edge cuts easily through armor and bone and leaves acid sizzling in the wounds it inflicts. The pain its wounds inflict is so severe that its victims have difficulty functioning, even walking, due to the agony. | ” |
Bugs[]
The item description states "+1d3 (acid)" as additional damage, where in fact it is 1d4.