House Celtigar horn
A Storm of Swords
After the Battle of the Blackwater, Ser Axell Florent urges Stannis Baratheon to attack Claw Isle. He tells Stannis that House Celtigar's castle is reputedly stuffed with treasurers, including a horn that can summon monsters from the deep.
Salladhor Saan berates Davos Seaworth, King Stannis's Hand, for preventing him from getting his hands on the riches of Claw Isle. Salladhor mentions that Lord Celtigar has a magic horn that can summon krakens from the deep which he would have found very useful. 
- Salladhor Saan, to Davos Seaworth