House Celtigar horn
The House Celtigar horn is a horn belonging to House Celtigar of Claw Isle. The horn is said to summon monsters or krakens from the deep sea.
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. 
...and a magic horn to summon krakens from the deep. Very useful such a horn would be, to pull down Tyroshi and other vexing creatures. 
- ↑ 1.0 1.1 A Storm of Swords, Chapter 36, Davos IV.
- ↑ 2.0 2.1 2.2 A Storm of Swords, Chapter 54, Davos V.