Lobstmourne is a One-Hand Fist Weapon dropped by Clawlord Kril'mandar, an NPC which you can summon with the Clamshell Band in Krasarang Wilds.

How to get the Clamshell Band

To make the Clamshell Band you will have to loot clamshells from six other makrura-type NPCs scattered throughout Pandaria.

/way The Jade Forest 59.8 95.8 Akkalar
/way The Jade Forest 59.2 36.6 Akkalou
/way Dread Wastes 27 69 Clamstok
/way Krasarang Wilds 39.4 88.6 Damlak
/way Kun-Lai Summit 42.4 24 Kishak
/way Kun-Lai Summit 54.2 21.6 Kishak
/way Townlong Steppes 42.6 92.6 Odd'nirok

Clawlord Kril'mandar

After obtaining all 6 items, you will be able to fuse them together to create the Clamshell Band. Then you can make your way over to Krasarang Wilds to summon Clawlord Kril'mandar.

/way Krasarang Wilds 12.6 82 Clawlord Kril'mandar

Upon defeat, the Lobstmourne will drop.

If you loot this item, bear in mind that there must always be... a Pinch King.