This achievement is gained upon defeating the following rare monsters in The Buried Sea.

Blacksail Bob
Blera`mar
Bloody Marc
Centehua
Cinderstorm
Coraline
Dreadclaw
Drowned Pirate Roberts
Friar Jake
Garayy Gali
Garayy Yuundu
Globmogger
Golden Ghost
Helushka
Iolana the Defender
Iuitl, the Prophet
Jelly Bones Bud
Lutzan, the Magma Reaver
Maatha Wirringan
Maleko, Sea Prophet
Manat, Archmage of Redfeather
Marta the Mad
Palgrave Cutler
Pythos
Reefrunner
Regrua Warcaller
Relentless Ruth
Researcher Aeglos
Rufus
Shellara Songweaver
Shertak
Shipwright Barney
Stonejaw
Teragadon
Terraraptor
Tetraton
Thunderstep
Velora Spinefin
William Hardcase